Noun: A player-character, typically in a MMORPG, that is able to occupy an agressor and take the brunt of the aggressor's attacks while the other players deal damage, heal, or perform some other action. The "tank" can survive longer while taking damage than the other characters. Typically, the tank has higher hitpoints (health) and a higher armor rating than the other characters. Less commonly, a character with a lower armor rating can perform the actions of the tank by using avoidance abilities. Ultimately, the tank needs to be able to occupy the agressor without dying. The method employed, whether avoidance or absorption, does not matter, as long as the tank can mitigate damage in some way.
Verb: To "tank", or "tanking", would be the action of performing the duties of the tank as described above.
1.N. That warrior is a good tank because he can take a lot of damage and he has a lot of hitpoints.
2.V. The warrior is tanking the mob right now.
3.V. Do you think you can tank this mob for us?