Sens 1 : a) existing as a separate indivisible entity; numerically one, single.
b) single, as distinct from others of the same kind; particular, special.[Source : OED]
Sens 1 : a particular being or thing as distinguished from a class, species, or
collection: as a single human being as contrasted with a social group or institution, a single
organism as distinguished from a group.[Source : MW]