Is "overflow" a term used to express "computed for every row"? I can see where the optimization would come from, when comparing `created_at` with a fixed value

Is "overflow" a term used to express "computed for every row"? I can see where the optimization would come from, when comparing `created_at` with a fixed value `now() - INTERVAL...` (assuming PostgreSQL is smart enough to evaluate it only once and reuse it for all the index comparisons), but the word "overflow" throws me out of the lane.