The Greek adjective heteroglōssos (ἑτερόγλωσσος) compounds heteros (another, different) and glōssa (tongue, language). It means "speaking another tongue, of a strange or foreign language." Paul cites it once in 1 Corinthians 14:21, quoting from Isaiah 28:11 to address the controversy over tongues in the Corinthian church.
Paul's use of heteroglōssos in 1 Corinthians 14 is deeply apologetic and pastoral. He pulls from Isaiah 28, where God declared He would speak to Israel through men of foreign tongues — and still they would not listen. The "sign" of strange tongues was historically a sign of judgment and hardness of heart. Paul applies this: tongues-speech without interpretation is unintelligible to unbelievers and edifies no one. The gift of glōssa must be ordered by love and intelligibility. Diversity of language is a fact of creation (Gen 11); the Spirit reverses Babel not by eliminating difference but by creating understanding across it (Acts 2).