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Wiktionary
unary operation

n. (context algebra English) An operation taking one operand like the square, factorial or absolute value operation.

WordNet
unary operation

n. an operation with exactly one operand [syn: monadic operation]

Wikipedia
Unary operation

In mathematics, a unary operation is an operation with only one operand, i.e. a single input. An example is the function , where A is a set. The function f is a unary operation on A.

Common notations are prefix notation (e.g. +, −, not), postfix notation (e.g. factorial n!), functional notation (e.g. sin x or sin(x)), and superscripts (e.g. transpose A). Other notations exist as well. For example, in the case of the square root, a horizontal bar extending the square root sign over the argument can indicate the extent of the argument.