sermo

Sermo, sermonis, m., third declension, from sero, "to join, to weave": words strung together.

Classically it is talk rather than oratory, and the contrast with oratio is the thing to hold on to. Oratio is a prepared public speech; sermo is conversation, everyday chat, and by extension the ordinary register of the language, so sermo cotidianus and sermo vulgaris mean what people actually said rather than what they wrote. Sermo Latinus is simply the Latin language. Christian Latin then took the word for the preacher's address, which is where English sermon comes from, and the Vulgate also uses Sermo to render the Logos, the Word.