From the "Counter this Pokemon"-thread
So, no. Check is not equal to a counter. Scizor is a check to Alakazam because it can KO it with a Bullet Punch, but it can not switch in 100% safely fearing an HP Fire and it will take massive damage from a Focus Blast. Jirachi is a counter as it can come in on any move and reliable KO Alakazam.-To be considered a counter, a Pokemon must be able to switch into any of the opponent's moves under normal battle conditions*, including taking damage from Stealth Rock, and reliably KO the opponent before the counter is KOed itself. (props to Woodchuck)
-To be considered a check, a Pokemon must be able to switch into at least one but not all of the opponent's moves under normal battle conditions, including taking damage from Stealth Rock, and reliably KO the opponent before the counter is KOed itself