alligator alligator /ˈæl.ɪ.ɡeɪ.tər/ aep /-t̬ɚ/ noun [C] (US INFORMALgator) ► F0 a large reptile with a hard skin that lives in and near rivers and lakes in the hot, wet parts of America and China. It has a long nose that is slightly wider and shorter than that of a crocodile.Thesaurus+: ↑Reptile and amphibian names