Not to be confused with "lswarm".
Swarming is the the act of quickly Summoning many monsters on the same turn. Archetypes with swarming capabilities include "Blackwing", "Raccoon", "Six Samurai" and "Synchron". Decks utilizing the Pendulum Summon mechanic are also capable of swarming.
Anti-swarm refers to cards that help to counteract swarming. Cards such as Mirror Traps, "Torrential Tribute" and "Dark Hole" are considered anti-swarm cards.
See also: Overextending