Instant

From MTG Salvation Wiki

Instants, like sorceries, represent one-shot or short-term magical spells. They are never put into the in-play zone; instead, they take effect when their mana cost is paid and the spell resolves, and then are immediately put into the player's graveyard.

Sorceries and instants differ only in when they can be played. Sorceries can only be played during the player's main phase, and only when nothing else is on the stack. Instants, on the other hand, can be played at any time, including during other player's turns and while another spell or ability is waiting to resolve. The term "spell" collectively refer to instants and sorceries.

From the Comprehensive Rules: 212.5. Instants 212.5a A player may play an instant card from his or her hand any time he or she has priority. Playing an instant as a spell uses the stack. (See rule 409, "Playing Spells and Activated Abilities.") 212.5b When an instant spell resolves, the actions stated in its rules text are followed. Then it's put into its owner's graveyard. 212.5c Instant subtypes are always single words and are listed after a long dash: "Instant - Arcane." Each word after the dash is a separate subtype. Instant subtypes are also called instant types. An instant subtype that's also a sorcery subtype is also called a spell type. Instants may have multiple subtypes. 212.5d Instants can't come into play. If an instant would come into play, it remains in its previous zone instead. 212.5e If text states that a player may do something "any time he or she could play an instant," it means only that the player must have priority. The player doesn't need to have an instant he or she could actually play.

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Card types
Groups	Supertypes	Card types	Subtypes
Spell (non-permanent)		Instant	Trap	Arcane
		Sorcery
Permanent	Basic, Legendary, Snow, World	Artifact	Equipment, Fortification, Contraption
		Creature	(...)
		Enchantment	Aura, Shrine
		Land	(...)
		Planeswalker	(...)
Miscellaneous		Tribal	(...)
		Plane	(...)
		Vanguard
Obsolete	Mono, Poly, Continuous	Interrupt, Mana Source