a solemn declaration: the binding pronouncement of a named speaker, used almost always as the prophetic tag "declares the LORD"
This is the word behind the phrase English Bibles render "declares the LORD," "says the LORD," or "oracle of the LORD." It is not ordinary speech but a formal, authoritative pronouncement, the kind of thing a sovereign issues, and it almost always carries a named source attached to it: this is the declaration of so-and-so. Roughly nineteen of every twenty times that source is God, which is why the word came to mean, in practice, the prophet's stamp certifying that what was just said is God's own word and not the prophet's invention.
The few places where the speaker is human do not change what the word means; they show its older shape before the prophets made it their signature. It introduces the inspired sayings of Balaam, the last weighty words of David, and the sayings of Agur. In every case it frames what follows as a measured, authoritative utterance, not casual talk. (Psalm 36 is the odd one: the speaker is David, but the thing declared is not God but transgression, which is why scholars read that line in several ways.)
Because the formula was so closely bound to genuine prophecy, it became a target for imitation. Jeremiah condemns false prophets who borrow the very phrase, wagging their tongues and announcing "declares the LORD" when the LORD has declared nothing, which shows how much weight the word carried and why counterfeiting it was so serious.