Game Design Terminology

Overpowered should be used to refer to game elements(character, class, vehicle, hero) that:
-Trivialize victory
-Are easily applied or executed
-Lack any possible counterplay within reasonable bounds (e.g. a character that is so powerful that they could not be beaten by any other character barring a large gap in skill between players)
Examples include Lion(R6S) (at release)

Overtuned should be used to refer to game elements that:
-Share a niche with other game elements (e.g. having multiple characters meant to fill the role of 'primary healer' on a team)
-Out-compete those other game elements by a wide margin (This might be indicated by a character being always picked over other options at all or most levels of play, not just lower tiers or pro league)
AND/OR
-Perform their task so well that counter-niche game elements lack the ability to reliably counter them
Examples include Tracer(Overwatch), Hibana(R6S)(Especially when she still had her claymore)

Undertuned should be used to refer to game elements that:
-Share a niche with other game elements
-Are out-competed or made obsolete by all or most of these options by a wide margin
AND/OR
-Perform their task so poorly that game elements intended to be vulnerable to their niche are unthreatened by them
Examples include Bastion(Overwatch)(on release)

Underpowered should be used to refer to game elements that:
-Severely undermine chances of victory by being taken
-Are extremely difficult to execute compared to the value gained by near-perfect execution
-Lack any reasonable application
Examples include Tachanka(R6S)