Define formula unit?

A ##"formula unit"## is in effect the ##"empirical formula"##, and it is particularly used to describe non-molecular species.

And the ##"empirical formula"## is the simplest whole number ratio that describes consitiuent atoms in a species.

Typically, we use ##"formula unit"## when we describe the composition of non-molecular entities such as salts. Common salt, sodium chloride, is certainly not molecular. Its description by its formula unit as ##NaCl## is unambiguous.