Originally Posted by Polaritie
Yes, it really is.
Tanks are basically hard to kill AND have a strong initiation move. Mord is hard to kill, but he's not a tank, because he has no initiation.
By contrast, Galio is hard to kill and has one of the game's best initiation skills.
Blitz is iffy if he's a tank, since Q is a great way to open a fight (Pull the carry, crush the carry)
You can also call champs like Garen and Morde a subset of tanks, or "offtanks", because they're a good choice for a second tanky character on a team if you feel that you need one, but as you say lack initiation or cc.
This differentiates them from champs who are hard to kill but whose primary function is actually dealing strong dps, like Olaf (though that again is different from being an actual dps carry
Anyway, this stuff is a bit semantic. If you know how to play Galio well, it doesn't matter whether you call him a tank or not. It's helpful to define the terms for facilitating out-of-game strategy discussion, but once the game starts it's a bit meaningless.