A tank is a character with a lot of health and damage reduction who either:
A) Must be attacked before you can attack someone else (ie: a taunt, but also through being a threat)
Or
B) Controls the other team so the can't do what they want to do.
Old Ben is a tank because he has a whole lot of health, protection, and Defense; he increases the survivability of his allies through dodge as a leader, and he controls the other team with Jedi Mind Tricks.
Fives is a tank because he's got the most defense and hp, counterattacks, and can call assists all day, so you have to take him out and suffer his counterattacks (eventually).
Teebo is DEFINITELY a tank, one who protects his allies by giving them stealth.
Boba Fett's a bad tank. No one's denying that. He's got the pre-requisite health and protection, and an AoE debuff, but not much else.
Likewise, Mace Windu is a tank because he has a lot of health and such, and instead of taunting he theoretically does so much damage that your opponent wants to prioritize him. He fails on that last account, though--his damage is good, but too random (can't control who Vaapad exposes) to be a real threat.
