Coordination Number
The number of nearest neighbours immediately surrounding a given atom or ion in a lattice or complex.
Unit
Dimensionless
In depth
In ionic solids the coordination number is fixed largely by the radius ratio of cation to anion: larger ratios accommodate more neighbours. In coordination chemistry it counts the donor atoms bonded to a central metal and, together with the metal's electron count, determines the geometry of the complex.
Examples in the real world
Each Na⁺ in rock salt touches six Cl⁻ ions; each Cs⁺ in caesium chloride touches eight.