Does flash allow you to cast a creature spell during an opponent's combat phase?

Flash is a keyword ability defined in CR 702.8a. Normally, creature spells can only be cast during your main phase when the stack is empty and you have priority (a 'sorcery-speed' restriction per CR 307.1). Flash overrides this restriction, allowing the creature to be cast any time you could cast an instant.

Instants can be cast at any time you have priority and the game is at an appropriate moment — including during any phase or step of any player's turn, such as an opponent's combat phase (CR 116.2b, 602.3a). So a creature with Flash, or any creature while a continuous effect grants it Flash, may be cast during combat.

A common strategic use is casting a Flash creature during the opponent's declare-attackers or declare-blockers step. For example, if your opponent swings with a 3/3 and you have Ambush Viper (a 2/1 with Flash and deathtouch) in hand, you can cast it during the declare-blockers step, immediately block the 3/3, and destroy it with deathtouch — all because Flash granted you instant-speed timing.

Keep in mind that you still need priority to cast spells (CR 116.1). During an opponent's turn, you typically receive priority after the active player passes it, so you must wait for the appropriate window. But once you have priority at any point during combat, Flash creatures are fair game.