1. A member of a legion or a group of soldiers or former soldiers: Historically legion was a unit of the Roman army of about 5000 soldiers.

Now legionnaire can refer to any large military force; including various veterans’ organizations that call themselves legions including the American Legion or The Royal British Legion.

In these modern times, a legion can mean a multitude or a significantly large number of people, things, etc.

2. Etymology: from Latin legere, "to gather" and from the Indo-European root leg-, "to collect".