noun

• A large, highly organized military force, concerned mainly with ground (rather than air or naval) operations.
  "The Fourth Army suffered such losses that its remainders were merged into the Second Army, also deployed on the Western front."
• (figuratively) A large group of people working toward the same purpose.
  "It took an army of accountants to uncover the fraud."
• (figuratively) A large group of social animals working toward the same purpose.
  "Our house is being attacked by an army of ants."
• (figuratively) Any multitude.
  "There was an army of construction cranes working on building the skyscraper."

noun

• (fandom slang, uncountable) The fandom of the K-pop boy band BTS.
  "ARMY are clever, too: they urged fans not to buy J-Hope's mixtape Hope World on its release date, having worked out that delaying by a day would result in a higher ranking in the Billboard charts."
• (fandom slang, countable) An individual fan of the K-pop boy band BTS.
  "Entering the theater, I expected the room to be filled with ARMYs decked out in their BTS merchandise."