population has 1 distinct synonym spread across 1 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the people who inhabit a territory or state
“the population seemed to be well fed and clothed”
a group of organisms of the same species inhabiting a given area
“they hired hunters to keep down the deer population”
(statistics) the entire aggregation of items from which samples can be drawn
“it is an estimate of the mean of the population”
the number of inhabitants (either the total number or the number of a particular race or class) in a given place (country or city etc.)
“people come and go, but the population of this town has remained approximately constant for the past decade”
the act of populating (causing to live in a place)
“he deplored the population of colonies with convicted criminals”
Synonyms, antonyms and the broader and narrower relations for population come from Princeton University’s WordNet. Full sources and licences.