When a raid boss in an MMORPG (such as World of Warcraft or EverQuest) has an ability that reduces the tank's aggro by a fixed percentage. Since a tank can only build up a certain amount of aggro in a fixed period of time, and since the boss performs this aggro-reducing ability periodically, there will be a maximum amount of aggro a tank can produce.
In WoW, this first appears on Broodlord Lashlayer in Blackwing Lair, since previous aggro-reducing bosses reduce a fixed amount of aggro, rather than a percentage. It also appears on the three drakes, Firemaw, Ebonroc, and Flamegor.
Other players (besides the tank) must be careful to control their aggro on aggro ceiling fights so as not to pull aggro, since they are not being hit with the aggro-reducing ability.
Careful to keep dps low when we're fighting Broodlord or you'll hit the aggro ceiling.