The Greek adjective eusēmos (εὔσημος) means clear, distinct, easily recognized, intelligible, having a plain meaning. Combining eu (good, well) and sēma (sign, signal, mark), it describes communication that sends a clear, unmistakable signal. It appears once in the NT — 1 Corinthians 14:9 — in Paul's extended argument about intelligible speech in the assembly.
1 Corinthians 14:9 states: "So with yourselves, if you utter speech that is not eusēmon, how will anyone know what is said? For you will be speaking into the air." Paul's argument is relentlessly pastoral and communicative: speech in worship must be intelligible to build up the assembly. He has just used the analogy of musical instruments (v.7-8) — if the flute or harp gives an indistinct sound, no one knows the tune; if the trumpet gives an unclear call, no soldier prepares for battle. Communication that cannot be understood defeats its own purpose. The Spirit's gifts are not for display — they are for edification. Eusēmos speech serves others; unintelligible speech serves only the speaker's sense of spirituality.