girl has 10 distinct synonyms spread across 4 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. Its one recorded opposite is boy.
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.
a youthful female person
“the baby was a girl”
a female human offspring
“her daughter cared for her in her old age”
a girl or young woman with whom a man is romantically involved
“his girlfriend kicked him out”
a friendly informal reference to a grown woman
“Mrs. Smith was just one of the girls”
Synonyms, antonyms and the broader and narrower relations for girl come from Princeton University’s WordNet. Full sources and licences.