Tank = heavily armored highly mobile vehicle used in open warfare susceptible to tight spaces and flanking. In other words braum riding on a corki.
... what's a tank?
The question is, which word?
"Tank" and "meta" are both rather frequently mis-used words.
I agree people are misusing the word "tank". Yasuo with tank items is not a tank. It's just op (ty keystone masteries).
HOWEVER, we are absolutely in a tank meta. Look at the highest winrate junglers for this season: Hecarim, Amumu, Sejuani, Zac, Skarner, Volibear. And in the toplane, maokai and malphite were pretty strong all season. And as for this bullshit tanksassin meta, while not true tanks, the meta was definitely shifted toward tanky damage dealers that could abuse the fervor and/or grasp of the undying keystone masteries.