A term for the smallest group of atoms that forms a repetitive pattern in a crystalline solid. In most cases the unit cell is either cubic or a hexagonal prism. It will usually have the center of an atom at each corner of the unit cell. For these atoms, only a portion of the atom counts as being in the cell, the rest of the atom would be in adjacent cell(s). They are in most cases symmetrical. A lot of information about a material can be learned from just it's unit cell, including density, and the atomic packing factor, which is the ratio of space occupied by atoms over the total volume.

The three most common unit cells are the Body Centered Cubic, the Face Centered Cubic, and the Hexagonal Close Packed.