Introduced as a keyword to Magic: The Gathering in 2006 with the Time Spiral expansion, Flash is a mechanic that doesn’t take long to understand.
Cards with Flash are ones that can be cast “any time you could cast an instant,” but there’s an important reason why those cards aren’t simply referred to as Instants.
An Instant in MTG is a spell type that’s comparable to a Sorcery spell. The difference is Instants can be played during another player’s turn or while another spell or ability is waiting to resolve.
Essentially, an Instant can be cast at just about any time. In contrast, Sorcery spells can only be cast on your turn during a main phase and cannot be used as a reactive measure if your opponent is trying to play a spell.
What makes the Flash mechanic relevant in MTG is that it extends the Instant ability of cards to card types that aren’t just short-term, Sorcery-like spells.
Flash is used primarily to help players instantly put creatures, enchantments, or artifacts onto the battlefield.
For the most part, cards that have Flash in them are blue because of the way that the mechanic synergizes with the “control” style of play the color is known for. There is a notable number of green and white cards that also have Flash, however. To a significantly lesser extent, some black and red cards have it as well.
Of the 292 cards that include Flash in MTG’s history, more than 40 percent of them include blue in some capacity.
More often than not, Flash is used as a mechanic for creature cards that have some sort of extra effect.
One example would be Spellstutter Sprite. While the Faerie Wizard is a creature card with Flying, it can be flashed onto the battlefield as a way to counter a spell that your opponent has played.
Additionally, there are some cards that give you the ability to cast other cards as though they had Flash. Some are meant to make Sorcery spells like Instants, and others are intended to give you a chance to summon creatures or other card types instantly.
In all, there are currently 22 cards that give players the ability to cast other cards as though they had Flash. Nine of those cards are creature cards, three are Enchantments, three are Instants, three are lands, two are Planeswalkers, and one is an artifact.