The attached contributions are helpful to understand difference between nano-crystalline and amorphous material.

Article Advances in amorphous and nanostructured materials

Article Amorphous and Nanocrystalline Materials for Applications as ...

Nano crystal periodicity of lattice only of one dimension or we can say a unique arrangement of atom in solid in one direction and long range order

amorphous not periodicity and short range order of lattice

To actually answer your question, nanocrystal is definitely

True that @Christian Kubel.

Nano-crystalline materials are subset of a larger group of materials that are classified as polycrystalline owing to the presence of grains with different crystallographic orientation, ............only that physicists called them crystallites (instead of grain) which brings us right to the deja vu of 'Crystallite Vs Grain" yet again.

This is a a very good question that I have come across a lot of times in my research. The nanoparticles I synthesized are in size ranges between 5nm to 30nm and the XRD measurement of the materials show a broadened peak around the ca. 45 degree for bcc Fe. The peak is also a combination of an amorphous/nanocrystalline phase and a crystalline phase. Now to confirm this further we have to do TEM and look for lattices and grains in the sample. As many researchers had stated here, the difference between nanocrystalline, polycrystalline is somewhat clear based on the orientation of the grains, but still does not completely answer the bigger question which is what is the difference between amorphous, crystalline, nanocrystalline and polycrystalline. What will they look like under TEM or how does the diffraction in x-ray measurement looks like. When we say that nanocrystalline are less than 1 uM then does that mean their xrd peaks are sharp or broad, or in other words what should the size of the crystallite be so that we can have a sharp peak in XRD. The high resolution TEM images show grain sizes of 3nm - 7nm and is this means they are amorphous or nanocrystalline or poly.

@Srinivasan Balakrishnan

As far as my understanding goes, your samples (3 - 7 nm in TEM) is polycrystalline material having a nanometer size crystallite distribution ranging from 3 - 7 nm, and the crystal range is just shy of being practically amorphous.

"Nano" (as the name itself suggest) is an attribute that comes about from its size.

A nanocrystalline material does NOT cease to be a polycrystalline material. Estimate of crystallite size above 100 nm with XRD is erroneous, that is roughly the range upto which the size broadening is discernible. Above 100 nm one is mostly or only left with instrumental broadening. Although it is most often guided by the properties rather the size, less than 1 micron is way too large, it should be less than 100 nm or smaller.

A rough guide to the nano to amorphous boundary can be grasp from the inverse Hall-petch with caution. Be also informed that the lower limit (by volume fraction) for detection of a crystalline phase by XRD is 5%. That means sample which is amorphous in XRD can exhibit crystalline phase in localized structural probing technique like a micro-Raman spectroscopy.

There is a lot difference between Nanocrstalline and polycrystalline materials. In nanocrystalline materials, the grains are in nanosize, that is a few nanometers to around 100 nanometers. These is no exact distinction of this numbers. In a polycrtstalline material, the gran size has no limts. You can calll the nanocrystalline materails as polycrystalline material but not otherwise.

Nanocrystalline materials tend to become polycrystalline materisl upon heat inf becasue of the growth of grains. similar way, polycrystalline materials can become nanocrystalline materials by ion bombardmentr ot mechanical atrition, etc.

Solid materials can be synthesized in crystalline form or amorphous/glassy form. Now, the crystalline material can be single crystal or poly-crystals, i.e. solid can be made up of one big single grain or a collection of grains adhered with each other with grain boundary inter-spacing. Now, in case of such poly-crystalline material, the size of one single grain can be at micron level or at nanometer level. Any grain with size less than 100 nanometer is considered to be nanograin. So solid polycrystalline material, which are consisted of grains with sizes less than100 nm are called nanocrystalline. Their individual grain size being small, are comparable to their grain boundary spacing, which considerable influence their material properties. Whereas, the poly-crystalline material which have grains in

poly-crystalline materials are nano-crystalline if their size are within the range of nano

certainly they have deference; polycrystalline is refers general terminology in which size of the grain  is not spesified, but more than one crystal. where as Nanocrystalline; the size of the grain  is in nanometer. The properties of Materials;  Mechanical proerties, electrical properties, ...,  also depende on Poly and Nanocrystalinity.      

perhaps the difference could be related to what the antonims are:

- polycrystal could be the antonim to monocrystal

- nanocrystal could be the antonim to the micro-crysta (it may be more related to "grain" size)

this means that the firs term could be linked to crystallinity and the second one to dimensions

Size matters!  It is well established that many physical properties are crystal size-related.  In magnetism, for example, the Curie temperature of nano-sized crystals, as well as their anisotropy constants can significantly differ between nano-scale and macroscopic sized objects.  Same goes for lattice constant.  Nano-sized crystals sometimes also exhibit unusual symmetries, like five-fold axes, "forbidden" at macro scale.  Therefore there is a good reason to distinguish such crystals as they usually differ from polycrystals.  All this applies to objects smaller than, say, 30 nm in diameter (the limit is not sharp).  Larger objects, but still below 1 micromater, are indeed called nanocrystalls - mostly for funding purposes.

few month ago we have a full-scale discussion of how one should define nano-scale effects and nano-materials. The number of good comments was high enough to publish a small book on this topic. I would suggest you to find this discussion on research gate, as it contain answers and arguments which are directly relevant to your case. But to make a long story short- it is only matter of definition.

crystalline materials can be classified into poly-crystalline and mono-crystalline, poly-crystalline are constituted of crystals oriented in different orientations, the single crystal materials are formed of a crystal with a direction of a plane. nano-crystalline materials are composed of grains of size nonométrico

Indeed, the polycrystalline structure is  a general descriptive structure of  of materials.   In contrast, the nano-crystalline structure is a relative descriptive structure of materials, relating  to the scale of  nanometer=1.0x10 -9 m.   In other words, 1m=109 nm, 1cm=106 nm, 1um=103nm , and so on.   However, the nanometer scale became a significant issue in materials science & solid state physics , especially, after the innovation of the scanning tunnelling microscopy (STM) in 1986, in which the atomic spatial resolution of condensed matter (A0=1x10-10 m) was achieved.   Consequently, since then, numerous works were carried out emphasising on the significant role of nano-polycrystalline in improving the properties of materials.   some of my early works on the above subject  are herewith  attached.