What is Muster?

Muster definition and meaning on Dictionary terms:
verb (used with object)
to assemble (troops, a ship’s crew, etc.), as for battle, display, inspection, orders, or discharge.
to gather, summon, rouse (often followed by up): He mustered all his courage.

verb (used without object)
to assemble for inspection, service, etc., as troops or forces.
to come together; collect; assemble; gather.

an assembling of troops or persons for formal inspection or other purposes.
an assemblage or collection.

the act of mustering.
Also called muster roll. (formerly) a list of the persons enrolled in a military or naval unit.

Verb Phrases
muster in, to enlist into service in the armed forces.
muster out, to discharge from service in the armed forces: He will be mustered out of the army in only two more months.