When there is scare of water during growth period of crop  stress occurs that disturbs physiological functions in plant. As a result there is reduction and no production of crop.

Dear Mr. Hebbar, The terms drought and water deficit is sometimes exposed as synnonymous. But I think water deficit is lack of enough water to continue physiological activity and drought is lack of water to continue physiological activity. Hope this would help you.

We can use the term that you ask, when environmentally be an scarcity of water in a determined period of time. But if you'll want use the term in agriculture, hydrology or any other area that involve the weather, you need define the variables that intervene in the area of your interest to define the correct term of drought. The drought generally was measured in severity index that defines the drought or humidity in the area of your interest.

I agree with Abdulla and Aliraza

Dear K.B.Hebbar,

I usually used drought as any  lack of water for seed germination, no matter how. But an important scientist once explained me that if my experiment was  with water restriction due to PEG or other osmoticaI should use the terms "osmotic stress" or "water deficiency", with a preference for the first. Dought stress I should use only in experiments in wich I really dont irrigated the plant, seedlings or even seeds. 

Hope it helps.

Both are synonyms although Water deficit is more commonly applied in scientific literature where it relates to water availability below field capacity and it is used especially in relation to crop cultivation. On the other hand drought is generally related to low precipitation over a period of time

Drought and water deficit stress both are related to the availability of soil moisture to the plant. Both may be called as "soil moisture stress" frequently faced by plants. This will vary according to the soil properties. As previously discussed, under drought situation, plants face inadequate water to drive their physiological functions, leading to leaf senescence  and other changes, while "under irrigated" plants face water deficit stress where enough water is not available for absorption and transport of solutes from soil, leading to growth retardation. Plants developed for deficit irrigated conditions should also have adaptability to nutrient poor environments. 

The  crispy definition given above by Dr.Abdullah Abdulaziz Al Doz is more accurate.

However the term drought is mostly coined in a larger term related to field drought due to the absence of either rain or  irrigation water. Therefore, the crop experiencing  stress resulting in poor or nil  growth and development. The word drought is coined if  water deficit  in both soil as well as atmosphere (vapour pressure deficit of  atmospheric air), therefore it is broader at ecosystem level. Drought is normally set in summer season in India with concomitant occurrence of high solar light intensity, practically no rainfall, high  temperature and high atmospheric VPD.

On the other hand water deficit stress is generally defined in the context of soil moisture deficit due to poor or less available water to the root system leading to stress in plants. There is no sense related to atmosphere,  it is just soil moisture deficit.

Plants experience water stress either when the water supply to their roots becomes limiting or when the transpiration rate becomes intense. Water stress is primarily caused by the water deficit, i.e. drought or high soil salinity. In case of high soil salinity and also in other conditions like flooding and low soil temperature, water exists in soil solution but plants cannot uptake it – a situation commonly known as ‘physiological drought’. Drought occurs in many parts of the world every year, frequently experienced in the field grown plants under arid and semi-arid climates. Regions with adequate but non-uniform precipitation also experience water limiting environments. Since the dawn of agriculture, mild to severe drought has been one of the major production limiting factors. Consequently, the ability of plants to withstand such stress is of immense economic importance. The general effects of drought on plant growth are fairly well known. However, the primary effect of water deficit at the biochemical and molecular levels are not considerably understood yet and such understanding is crucial. All plants have tolerance to water stress, but the extent varies from species to species. Knowledge of the biochemical and molecular responses to drought is essential for a holistic perception of plant resistance mechanisms to water limited conditions in higher plants.

I suppose 'drought stress' is more commonly used and is certainly more straightforward than 'water deficit stress'.  I would like to give a word of warning with regard to related terms though: 'water stress' or 'moisture stress' are highly ambiguous terms and shouldn't be used synonymous to 'drought stress'. Reasoning: The term 'water stress' itself doesn't provide a clue whether one is talking about stress effects resulting from too much or too little water, i.e. plants subjected to flooding could rightly be called 'water stressed' as well. Therefore, to prevent any ambiguity the term 'drought stress' should be used when plant stress is due to low soil water availability, high VPD etc.

Physiological drought is said to exist when the plant suffers a water deficit (i.e. low tissue water potential) caused by salinity, low soil water potential or other stress factors (Fuchigami et al., 1996). 

Water deficit (commonly known as drought) can be defined as the absence of adequate moisture necessary for a plant to grow normally and complete its life cycle (Zhu, 2002). The lack of adequate moisture leading to water stress is a common occurrence in rainfed areas, brought about by infrequent rains and poor irrigation (Wang et al., 2005).

The term "drougt" indicates a deficiency of water in an environmental or global sense, i. e. when in a given region of the Earth a lack of water for a prolonged period of time (the region receives less water than its natural supply) occurs. The term "water deficit stress" implies that the lack of water is such as to cause damage to that particular organism, for that particular metabolic function or specific for that particular place, etc. in a word in a more specific and less global sense. This is my opinion and experience of use of drought and water deficit stress; I hope to be helpful.

So a salt stress can lead to 'water deficit stress' but not 'drought'?

 Salt stress secondary effect leads to water deficit condition and hence stress 

Drought can be defined as the absence of adequate moisture necessary for a plant to grow normally and complete its life cycle. Drought is intensively documented to alter almost all metabolic pathways occurring in the plant cells, and thus it can modify plant morphological, ultrastructural, physiological, biochemical and molecular features (Demirevska et al., 2008). The response of plants to drought stress varies greatly depending on the intensity and duration of stress, genetic makeup of plant species and its stage of growth (Parameshwarappa and Salimath, 2008).

The pattern of water use is crucial for plants grown with a limited amount of water in soil profile because the plant success depends largely on a sustained water use (Kato et al., 2008; Aldesuquy et al., 2014a). However, plants are developmentally and physiologically designed by evolution to reduce water use under drought stress (Blum, 2005). Therefore, the study of water use efficiency (WUE), defined as the ratio of dry matter production to water use, is particularly interesting in situations where growth is affected as a result of limiting water availability (Anyia and Herzog, 2004). The effect of drought on WUE has been investigated in different plant species such as Zea mays (Ashraf et al., 2007).

Leaf relative water content (RWC) is proposed as a more important indicator of water status than other water potential parameters under drought conditions, as it is believed that RWC is a reliable parameter for quantifying the plant-drought response (Rahbarian et al., 2011). During plant development, water deficiency significantly reduces RWC (Siddique et al., 2000). A decrease in RWC in response to water deficit had been reported in several studies (Aranjuelo et al., 2011; Farshadfar et al., 2012; Hasheminasab et al., 2012). As a practical proof, Shinde et al. (2010) observed that drought considerably reduced RWC in four groundnut varieties. Additionally, water-stressed wheat and rice plants had lower relative water content than non-stressed ones (Farooq et al., 2009). 

Water-deficit stress  defined as a situation in which plant water potential and turgor are reduced enough to meet with normal functions. While drought stress occurs when  the demand of plants exceeds the supply by (rainfall or irrigation) at any stage of plant growth.  

I concur to the observation that the use of the term "moisture or water stress" is ambiguous but rather "water deficit stress" as flooding, when water supply is in excess, can also constitute stress. To be specific, water deficit stress or drought stress should be used when supply of moisture is below the quantity required by plant for optimal performance.

Conceptually though both terms seem similar they are different. Water deficit is about the state when the supply of water to the plant could not meet its demand. Drought also displays similar pattern, however there is the added increase in temperature. The strain response in both cases is dehydration. The difference could be the causative factor in dehydration, just as osmotic stress would elicit dehydration together with ion toxicity. Probably a better way of making clarity between both concepts could be that drought would elicit vapour pressure deficit in the atmosphere, while water deficit is more through the soil.

In my opinion, drought occurs when plants experience water deficit to the point that prevents it from functioning normally. Precisely, water-deficit here means a reduction in plant water potential. Water deficit can be due to several different factors such as heat, soil dryness, salinity, etc. But we need to differentiate between and . I would say that DROUGHT is a 'case/situation' that exists in the field or the surrounding area, whereas WATER-DEFICIT is the 'effect factor'. In this case, soil dryness will be the 'causer' for both the 'case' and the 'effect factor'. In arid and semi-arid regions, for example, someone can say that drought occurs due to soil and/or atmospheric water-deficit. The situation (or treatment) is a drought of the surrounding area; the effect (or variable) factor on the plant is the low water potential, and the causer for both, the situation and affect factor, is the water-deficit in soil and/or air relative water. It is probably important to mention here that low water potential of a plant (leaf) results in dehydration, whereas severe water-deficit leads to desiccation. All these terms (drought, dehydration, and desiccation) are not synonymous. This is my concept of the ambiguity between drought and water-deficit, and I hope it is helpful.

Dear K. B. Hebbar

I think this paper can help you

Article Screening of different Iranian ecotypes of cannabis under wa...