unit cell

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The smallest building block of a crystal, consisting of atoms, ions, or molecules, whose geometric arrangement defines a crystal's characteristic symmetry and whose repetition in space produces a crystal lattice.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mineralogy In a crystal, the smallest repeating structure (parallelepiped) of atoms from which the structure of the complete crystal can be inferred.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the smallest group of atoms or molecules whose repetition at regular intervals in three dimensions produces the lattices of a crystal