Entry 1 sense Webster, 1913 antimatter /(ăn`tĭ*măt"tẽr)/ · an·ti·mat·ter · IPA /ˈæn.tiˌmæt.ɚ/ 01 n. Matter whch is composed of antiparticles such as antiprotons, positrons, and antineutrons. 1. Matter whch is composed of antiparticles such as antiprotons, positrons, and antineutrons.(Physics)