King James Bible Dictionary

A regiment of the Roman army, the number of men composing which differed at different times. It originally consisted of three thousand men, but in the time of Christ consisted of six thousand, exclusive of horsemen, who were in number a tenth of the foot-men. The word is used (Matthew 26:53; Mark 5:9) to express simply a great multitude.

Of angels
Matthew 26:53

Of devils
Mark 5:9; Mark 5:15

the chief subdivision of the Roman army, containing about 6000 infantry, with a contingent of cavalry. The term does not occur in the Bible in its primary sense, but appears to have been adopted in order to express any large number, with the accessory ideas of order and subordination. (Matthew 26:53; Mark 5:9)

LE'GION, noun [Latin legio, from lego, to collect.]

1. In Roman antiquity, a body of infantry consisting of different numbers of men at different periods, from three to five thousand. Each legion was divided into ten cohorts, each cohort into ten companies, and each company into two centuries.

2. A military force; military bands.

3. A great number.

Where one sin has entered, legions will force their way through the same breach.

My name is legion for we are many. Mark 5:9.

LE'GIONARY, adjective

1. Relating to a legion or to legions.

2. Consisting of a legion or of legions; as a legionary force.

3. Containing a great number; as a legionary body of errors.

LE'GIONARY, noun One of a legion.