unit cell

The smallest, regularly repeating material portion contained in a parallelepiped from which a crystal is formed by parallel displacements in three dimensions.
  1. Unlike in the case of low molar mass substances, the unit cell of polymer crystals usually comprises only parts of the polymer molecules and the regularity of the periodic repetition may be imperfect.
  2. In the case of parallel-chain crystals, the chain axis is usually denoted by c or, sometimes, b.
  3. This definition applies to the so-called primitive unit cell. In practice, the effective unit cell may consist of more than one primitive unit cell.
Purple Book, 1st ed., p. 76 (http://old.iupac.org/publications/books/author/metanomski.html)