clos·er (klō′zər) n. 1. One that closes: The closer of the shop has to lock up. 2. Baseball A relief pitcher called upon to protect a lead late in a game. ThesaurusAntonymsRelated WordsSynonymsLegend: Switch to new thesaurus Noun 1. closer – a person who closes something; “whoever is the closer has to turn out the lights and lock up” individual …