To state or declare something as true, especially in a confident or formal way; to assert or maintain.
From Old French 'averer' (to confirm), derived from Latin 'ad verum' (to the truth). Entered English in the 14th century as a somewhat formal or legal term for making a statement of fact.
You'll mostly hear 'aver' in legal documents or Victorian novels—it's one of those words lawyers love because it sounds more formal and serious than just saying 'claim' or 'say,' giving weight to someone's statement.
Complete word intelligence in one call. Free tier — 50 lookups/day.