I think MK is right about unary. It is less common than some mathematical words that are rightly classed as rare. (Surd was once treated as common, but I made it rare in 2009.)
Unary might be familiar to scholars of some of the more abstract branches of mathematics or formal logic. Negation of a number is a unary operation, because only one number is involved, whereas subtraction is a binary operation. In logic, NOT is a unary operation, while OR is binary.
Needless to say, it is not truly a common word, so it will be rare from now on.