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I define tanks in this game to be any champ built to:
1. Enter and/or initiate team fights
2. Have the endurance to last within a team fight - First in, Last out
3. Have the abilities to disrupt the enemy team's team fight plans
I play tank Singed quite a bit and feel that his Mega Adhesive is effective at splitting up teams, miring teams, cutting off enemy retreat, and aiding teammate retreat. His Fling ability lets me interrupt annoying ults (Katarina, Nunu, etc..), throw an enemy carry into my teammates to be eaten, throw enemies away from teammates so they can escape, throw their tank out of the fight (and over a wall into the jungle if they're standing in the wrong spot at the right time), and get the occasional kill. Poison gas is really just icing on the cake with a little damage and a lot of farming capability. Ultimate gets you in or out fast, provides great regen, and when combined with Ghost makes you so disgustingly fast you wonder why it's legal.