Boyd’s Bible Dictionary:
(gathered). Division of Roman army; when full, 6200 men and 730 horse. N. T. use indefinite (Matt. 26:53; Mark 5:9).
Concise Bible Dictionary:
In the Roman army a body of troops consisting of from three to five thousand; but the term is also used for an indefinite number. The Lord said that His Father on His request would send Him more than twelve legions of angels (Matt. 26:53). The demons who possessed the man among the Gadarenes said, “My name is Legion; for we are many” (Mark 5:9,15; Luke 8:30).
Strong’s Dictionary of Greek Words:
Meaning:
of Latin origin; a "legion", i.e. Roman regiment (figuratively)
Potts’ Bible Proper Names:
A legion:―a division of Roman army, Mark 5:9. {Legio}