A link is when a monster you are fighting causes nearby monsters to join in the battle. For example, if you're fighting a Goblin that is too weak to be worthwhile, any Goblins nearby may join in the battle, "linking" with the first. Some mobs may be aggressive but not link (e.g. Ironshell), and some may link but not be aggressive (e.g. Strolling Sapling).
May also be referred to as an Add, or BAF (Bringing a friend).
Some monsters display "superlinking" behavior: if they have no targets on their own hate list, they will link with their friends anywhere in the zone. For instance, most BCNM enemies superlink with each other, and Dynamis - Xarcabard mobs superlink with the statue or eye that spawned them. Also Dragoon Wyverns superlink with their masters.
It is more difficult to sac pull in the presence of superlinking: the sac puller must stay alive until the single pulled mob is killed. If the mob dies first, the rest of the mobs will deaggro when the sac puller dies. If the sac puller dies first, the rest of the mobs will link with the mob being fought, typically causing a wipe.