An Evolutionarily Signifcant Unit, or ESU, is a term used to to describe a population of organisms that is distinct for conservation purposes. This term was first coined by Ryder in 1986 and originally used by zoos as a method of classifying a group of organisms below the species level.
Ryder OA (1986) Species conservation and systematics: the dilemma of the subspecies. Trends Ecol Evol 1:9–10