Look up muster in Wiktionary, the free dictionary.
In military organization, the term muster is the process or event of accounting for members in a military unit. This practice of inspections led to the coining of the English idiom pass muster, meaning being sufficient. When a unit is created, it is "mustered in" and when it is disbanded, it is "mustered out". If a unit "musters" it is generally to take account of who is present and who is not.
A muster roll is the list of members of a military unit, often including their rank and the dates they joined or left. A roll call is the reading aloud of the names on the muster roll and the responses, to determine who is present.