An argument that depends on an assumption that is never made explicit — and which the audience would likely reject if they recognized it.
"We need stronger border enforcement to protect working-class wages." Unstated premise: immigration consistently and significantly depresses working-class wages. This is an empirically contested claim with a complex research literature. The argument skips over it entirely, treating it as obvious — because if it were stated explicitly, it would require defense.
Arguments feel complete when they move from premise to conclusion without visible gaps. We don't naturally inventory the unstated assumptions — especially when those assumptions align with pre-existing beliefs. The gap is invisible precisely because we fill it automatically.
Ask of any causal or prescriptive claim: "What has to be true for this conclusion to follow from these premises?" If the answer is a contested empirical claim, that's the unstated premise.