The article examines the original meaning and semantic development of the adjective ἠεροειδής in poems by Homer and Hesiod. The linguistic analysis of hypotheses concerning the etymology of the noun ἀήρ, from which this adjective derived, indicates that the lexeme ἀήρ was originally associated with the conception of invisibility. Сontextual analysis of Homeric expression ἐς ἠεροειδέα πόντον allows concluding that in Homer’s poems, the adjective ἠεροειδής meant “foggy”. But Hesiod for the first time used this adjective to describe night darkness thus contributing to its reinterpretation, and the lexeme ἡεροειδής acquired the meaning “dark”.