To call (troops) together, as for inspection. To cause to come together; gather: Bring all the volunteers you can muster. To call forth; summon up: mustering up her strength for the ordeal. See Synonyms at call. To assemble or gather: mustering for inspection. A gathering, especially of troops, for service, inspection, review, or roll call. The persons assembled for such a gathering. A muster roll. A gathering or collection: a muster of business leaders at a luncheon. A flock of peacocks. See Synonyms at flock1. muster in To enlist or be enlisted in military service: She mustered in at the age of 18. muster out To discharge or be discharged from military service: He was mustered out when the war ended. pass muster To be judged as acceptable.