HCP is one of the first abbreviations you’ll learn in materials science, but if you’re trying to learn materials science on your own, you may not know what HCP means.
HCP stands for Hexagonal Close-Packed, which is a type of crystal structure. The HCP unit cell has an atom at each corner of a hexagonal prism, and three more inside the prism.
You can build a hexagonal close-packed structure for yourself by placing hexagonal sheets on top of each other.
If you stacked the sheets as A-B-A-B you will get the HCP structure. If you stacked the sheets as A-B-C-A instead, you will get the FCC structure.
Unlike FCC and BCC, HCP does not share its name with a Bravais lattice.
HCP has an underlying hexagonal Bravais lattice, but it has extra atoms to make a 6-atom basis.