What does it mean to clash?

Clashing in Magic: The Gathering refers specifically to the action of a player revealing the top card of their library. This is part of a keyword ability that first appeared in the “Betrayers of Kamigawa” set, which involves a comparison of the revealed cards to determine the winner based on the card's mana cost. The process begins with a player revealing the top card of their library, and the outcome can lead to various effects depending on the context of the clashing ability.

The other provided options do not accurately describe the action of clashing. While revealing cards is a common mechanic in MTG, only the action of revealing the top card of the library directly connects to the clash mechanic.