The term 'A Single UNIT' can be represented by two crossword solutions: MONAD (5 letters) and ONE (3 letters). The term 'MONAD' is derived from the Greek word for 'UNIT' and is used in philosophy to refer to an indivisible, ultimate element of reality. It is also used in mathematics to refer to a single element of a set. The term 'ONE' is used to refer to a single UNIT or entity, and is often used to refer to a single person or thing.