Legion has 9 different meanings across 3 categories:
Noun · Adjective · Proper Noun
The major unit or division of the Roman army, usually comprising 3000 to 6000 infantry soldiers and 100 to 200 cavalry troops.
"The victorious general led his entire legion across the battlefield to secure the province."
In plain English: A legion is an extremely large number of people, often used to describe something so big it feels endless.
"There is an entire legion of fans waiting outside to see their favorite band perform live tonight."
To form into legions.
"The defeated army was scattered and forced to be legioned out across the border regions."
Numerous; vast; very great in number
"The legion of stars stretched across the night sky, creating a breathtaking display of countless lights."
In plain English: When something is described as legion, it means there are so many of them that you can't even count how much there is.
"The legion of ants marched across the picnic blanket in search of crumbs."
Usage: Use "legion" as an adjective before a noun to emphasize that something is extremely numerous, often implying the sheer scale of Roman military forces. This term functions hyperbolically rather than literally and should not be confused with its singular countable meaning when referring specifically to army divisions.
Ellipsis of American Legion
"He joined the organization immediately after his discharge from service."
The word legion entered English around 1200 via Old French and Latin, originally referring to a military unit gathered or collected. Its modern sense of meaning "a large number" comes from the biblical phrase in Mark 5:9 where demons call themselves Legion because they are many.