In general, crystallinity is the lost of crystal perfection. Nevertheless, in practice there are many criteries about this term, and many ways to determine it. Some scientists associate crystallinity with degree of amorphization, but other associate it with degree of order, concentration of puntual defects, and even with the empirical broadening of certain diffaction peaks. What really is crystallinity in theory and in practice from your point of view?
Crystallinity is a term used in materials which are not 100% crystalline, and denotes the fraction of the material which is crystalline (the rest being amorphous). That's a relative simple definition. The only reason it gets complicated is because it is very difficult to measure. A typical measurement will give a signal from the crystalline fraction, and a signal from the amorphous fraction, and the trick is to deconvolve the two and to conver the ratio of the signals into actual concentration ratios (which is complicated by the fact that x% amorphous may give more signal than x% crystals). Usually therefore either a calibration is needed or a "relative" crystallinity is used.
Techniques that spring to mind where amorphous and crystalline signals can be detected are Raman, XRD and FTIR, but there are probably others too.
I assume that your question is about "POLY-crystallinity", which is simply an agglomeration of small crystals or grains which all have the same arrangement of atoms within the grain, but the grains themselves have no crystallographic relationship with other grains.
Crystallinity is a term used in materials which are not 100% crystalline, and denotes the fraction of the material which is crystalline (the rest being amorphous). That's a relative simple definition. The only reason it gets complicated is because it is very difficult to measure. A typical measurement will give a signal from the crystalline fraction, and a signal from the amorphous fraction, and the trick is to deconvolve the two and to conver the ratio of the signals into actual concentration ratios (which is complicated by the fact that x% amorphous may give more signal than x% crystals). Usually therefore either a calibration is needed or a "relative" crystallinity is used.
Techniques that spring to mind where amorphous and crystalline signals can be detected are Raman, XRD and FTIR, but there are probably others too.
Fort example let's look at UHMWPE, it may be partially crystalline because of special stacking order of the PE chains. The method to detect the degree of crystallinity is differential scaning calorimetry , which measures the heat flow and mass changes in studied and standard samples. Calculating a ratio between melting enthalpy for standard 100% crystalline PE and that of studied one we get the degree of crystallinity. This method works best as compared to XRD.
While In usual polycrystalline materials the number of "non-crystalline" atoms at grain boundaries is less than 10%, in nanocrystalline materials this fraction can become more than 50%, see our old, but fundamental article: https://www.researchgate.net/publication/233858912_HREM-Studies_of_the_microstructure_of_nano-crystalline_Palladium
Of course the fraction of non-crysatlline atoms has an influence on the properties, nano-particles become more reactive, and much more ...
Article HREM-Studies of the microstructure of nano-crystalline Palladium
From my point of view, the "components" of the lost of crystallinity are (not in order of importance): a) crystallite size, b) mosaic structure, c) point defects, c) dislocations, d) surface defects (grain boundaries), e) volumetric defects (twinings, cavities, strains...), f) short range loss of order (amorphization), g) thermal motions. Perhaps the better way for determining the crystallinity is to relate these factors with the overall increment of entropy, and to find the experimental ways to determine each.
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