A verbum dicendi (or declaratory word) is a word that expresses speech, introduces a quotation, or marks a transition to non-standard or non-grammatical speech. Typically it is a verb, e.g. "say", "avow", "claim", etc. Occasionally, it may take the form of a copulative phrase, as in the colloquial sentence He was like "Turn down the music!", and I was all "No way!".
A verbum dicendi may theoretically take any form, provided that it introduces a quote or paraphrase. In some forms of informal English, it may even take the form of an action verb like "go", as in a sentence like So John goes "This sandwich is too big!"
A verbum dicendi may sometimes be used to obliquely introduce an ideophone or other non-standard word. For example, in the sentence The rabbit goes hippety-hop, the word "goes" is a verbum-dicendi.