# What is the difference between "formula unit" and "empirical formula"?

A $\text{formula unit}$ is in effect the $\text{empirical formula}$, and it is particularly used to describe non-molecular species.
And the $\text{empirical formula}$ is the simplest whole number ratio that describes consitiuent atoms in a species.
Typically, we use $\text{formula unit}$ 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 $N a C l$ is unambiguous.