A keyword ability that lets a card be cast at instant speed, i.e. any time a player could cast an instant, including during an opponent's turn. Flash creatures and spells are prized for ambushing opponents and holding up interaction without losing tempo. Flash is a static ability that functions in any zone from which you could play the card.
For new players
Multiple instances of flash on the same card are redundant.
Sphinx of False Conclusions has flash, so a player may play it any time they could cast an instant.