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.
C&C Work | 1k RMT | Contribute! | VM for an OU Rate! | gp member: vm/pm for a check | previously pokemon0078 / aka jew-cane