In mathematics, a unit is always alone (“single”). However, the word “unit” may be used to replace the name of an appliace or piece of equipment, such a compressor, an A/C appliance, etc. In this case, when only one “unit” is required or equipped, instead of saying, for instance, “single compressor,” it is common to say “single unit.”
pleonasm: 1. the use of more words than are necessary to express an idea; redundancy. 2. an instance of this, as free gift or true fact. 3. a redundant word or expression.
