A Tank is an RPG term used to refer to a character class within a party who is built to take damage and intended to be the target of attacks, thus protecting other members of the party. It can also be used as a verb, as in, "I'll summon up some minions to tank this big boss monster." This does not necessarily have to be the "tank" character saying it, anyone can tank, just not as good as "the tank".
This term is most often used in an MMO setting, although is it also used in certain single player RPGs where the single player controls a party.
It's generally used with other generic roles, such as a ranged player with DPS, some sort of caster and a healer.
These terms can equally apply to monster groups as well as player groups and summoned creatures, like minions or pets.
There is no particular "tank" class in Diablo III, like in many other RPGs. The closest thing would be the Barbarian, who gets benefits from taking damage.
In Diablo II, the Barbarian and the Paladin are more "tank-y" classes than the rest, although the Druid can take on this role, and both the Necromancer and the Amazon can summon followers to act as tanks.
In Diablo I, the Warrior is the only proper tank class before the expansion Diablo: Hellfire. In Hellfire the Monk can act as a tank, as can the secret Barbarian class.
- A tank is also an armored and tracked vehicle used in heavy combat usually with a main cannon on a swivel, and often secondary cannons or machine guns elsewhere. This is where the term Tank (as an RPG term) comes from because the armored military tank can take a lot of abuse and protect infantry taking refuge behind it.