Wiki gives a start: Affective states are psycho-physiological constructs. According to most current views, they vary along three principal dimensions: valence, arousal, and motivational intensity. Arousal is objectively measurable as activation of the sympathetic nervous system, but can also be assessed subjectively via self-report. Arousal is a construct that is closely related to motivational intensity but they differ in that motivation necessarily implies action while arousal does not.[5] Motivational intensity refers to the impulsion to act.[6] It is the strength of an urge to move toward or away from a stimulus. Simply moving is not considered approach motivation without a motivational urge present.[7] Valence is the subjective positive-to-negative evaluation of an experienced state.

Affect (psychology) - Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Affect_(psychology)

If you read some research articles, the authors will tell you what they use as an operational definition.

There is a helpful online outline on emotion at http://uwf.edu/jgould/Emotion%20What%20is%20it.pdf

In a nutshell, affect deals with the state of your emotions and/or moods. There is a stable aspect to it and it is also influenced by situational characteristics. Typically, affective states are separated into positive and negative affect because they are almost entirely independent of each other (not opposite ends of a single continuum). Highly positive affective states include enthusiasm, confidence, happiness, alertness, etc. Highly negative affective states include anger, disgust, fear, guilt, etc. And, of course, you can also have low positive (e.g. distress) and low negative (e.g. calmness). Affect factors into decision contexts because the affective state can be interpreted as a sort of signal to the decision maker. For example, I've read a recent paper that found that negative affect triggers greater persistence and thus better solutions to problem solving because the negative emotion/mood signals that the situation is not yet ideal and needs to fixed. Hope this helps.

Thanks all.  Brent the paper that found negative affect triggers greater persistence and solutions is a very helpful thought.

Affect is a psycho-social construct that triggers motivation . Values, attitude and perception and mood may also get linked to it. It is a deeper state of mind that may have intrinsic component. Anything with intrinsic component makes a drive stronger. It may make one very confident to complete a job again but you may feel demotivated , and unless pushed to the wall , you better shy away. Stimulus and response may be determined by the level of affect component. 

Hendrika Vande Kemp Thanks for your response, In the text mentioned that "Arousal is objectively measurable as activation of the sympathetic nervous system, ...." do you know how measure arousal from SNS?