From what I understand of the term 'Tank', it is a specific role found within RPGs (especially MMOs). Their job is to go in, take the pressure of their team mates by drawing the enemy to them (sometimes through an aggro system, sometimes through class abilities etc), and then sit and take damage (and the important bit:) without dying (hence high defenses).
The ability to deal out damage is usually less important, as they're often paired with high DPS (damage per second) builds. Although Tanks can, in some games, also deal out damage - if nothing else, it stops them getting bored.