A region whose translates tile a lattice's ambient space, together with the lattice points represented in it. A primitive unit cell represents one lattice point; a conventional unit cell may represent several to display symmetry.
A crystallographic unit cell chosen to display the symmetry or a standard setting of a crystal structure. It tiles the periodic arrangement but need not be primitive. A conventional FCC cube contains four lattice points and a BCC cube contains two; a primitive unit cell contains one. These are lattice-point counts, not a universal count of atoms in a chemical motif.
A fundamental cell of a Bravais lattice, of volume equal to the absolute determinant of any primitive lattice basis. Direct and reciprocal lattice primitive cell volumes multiply to in dimension with the angular-wavenumber convention.
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A unit cell is the smallest repeating unit of a crystal lattice that retains the geometric and symmetry characteristics of the entire crystal structure. It serves as the foundational building block from which the entire crystal structure can be constructed through translational repetition in three-dimensional space.