1. <programming> (or "monadic") A description of a function or operator which takes one argument, e.g. the unary minus operator which negates its argument. The term is part of the same sequence as nullary and binary.
2. <data, humour> Base one. A number base with only one digit, namely zero, and which can therefore only be used to express the number zero. Attempting to add one to zero results in an infinite sequence of carries. Numbers in unary notation can be represented particularly efficiently however since each digit requires no storage.