Definition:Operation/Unary Operation

Definition
A unary operation is the special case of an operation where the operation has exactly one operand.

Thus, a unary operation on a set $S$ is a mapping whose domain and codomain are both $S$.

Example
An example of a unary operator from algebra is $\sqrt{}$ (i.e. the square root sign).

Linguistic Note
Unary is pronounced yoo-nary.