Entry 2 senses · 2 variants Webster, 1913 Prophylactic /präfˌ-ĭl-ăk'-tĭk/ · Proph·y·lac·tic · IPA /pɹɒfəˈlæktɪk/ n. a. 01 n. A medicine which preserves or defends against disease; a preventive. 1. A medicine which preserves or defends against disease; a preventive.(Med.) 02 a. Defending or preserving from disease; preventive. 1. Defending or preserving from disease; preventive.(Med.)