Taxonomy: Classification, especially in relation to its general laws or principles. (OED)
The world has far too many things in it to treat them all individually. One way we make progress is through taxonomy. We group things so as to be able to discuss them usefully and understand them. The best-known taxonomy is that of living things: we group them as mammals, reptiles, grasses, trees. We group crystals based on their symmetry: cubic, hexagonal, orthorhombic.
Grain boundaries remain one of the main outstanding problems in materials science. We understand crystals rather well, but we do not have a good science of grain boundaries yet. It would help to have a taxonomy of grain boundaries.