[Jukain]

okay so i've got some definitions for you:

counter: a pokemon that can handle every possible set of a pokemon, every possible variation, and has the ability to retaliate adequately

of course there are counters to single sets, but that is the best definition imho.

check: this is a bit broader, there are two types basically:
switch-in: a pokemon that can switch into one or more attacks of a pokemon, notably its primary stab, and outspeed or has priority to ko the pokemon from there or shut them down a la taunt thundurus vs hazards deoxys-s

revenge killer: a pokemon that can't necessarily switch in but can outspeed or has priority or can shut down a pokemon

these definitions account for bulky pokes which i see others don't.