What does it mean to "counter" a spell or ability?

To "counter" a spell or ability means to cancel it and remove it from the stack. When a spell is countered, it does not resolve or have any effect, effectively nullifying its intended purpose. In Magic: The Gathering, spells and abilities utilize the stack, where they wait to resolve in a last-in, first-out manner. If a spell is countered, it never resolves and does not produce any effects, leaving the game state unchanged relative to that spell.

When considering the other choices, enhancing effects or increasing power does not align with the concept of countering; such actions typically involve a different set of interactions within the game. Returning a spell to its owner’s hand is also a different action, typically associated with spells or abilities that put things back into a player's hand rather than countering them. Switching control of a spell or ability would imply transferring its ownership, which is distinct from canceling its implementation. Countering fundamentally focuses on preventing a spell or ability from having any effect in the game.