Historical Rules · 1 October 2009
Rule 120.1
Official source text from the 1 October 2009 Rules release
A counter is a marker placed on an object or player that modifies its characteristics and/or interacts with a rule, ability, or effect. Counters are not objects and have no characteristics. Notably, a counter is not a token, and a token is not a counter. Counters with the same name or description are interchangeable.