What is a sideboard in a Magic tournament?

In the context of a Magic tournament, a sideboard refers to a set of additional cards that a player can utilize to modify their main deck between games within a match. Players have a designated number of cards—usually up to 15—that they can use to adjust their strategy based on their opponent's deck and gameplay. This allows players to tailor their decks to better counter specific threats or synergies they may face in subsequent games after having seen their opponent's cards and strategy during the first game.

This flexibility is a crucial part of tournament play, as it enables players to increase their chances of success by adapting to their opponent's strategy. A sideboard is not a collection of banned cards, nor is it the main deck that remains unchanged throughout the tournament. Additionally, the cards in a sideboard are intended to support the gameplay rather than serve as cards that cannot be used, making the first choice the accurate definition of what a sideboard is.