Spell

In Magic: The Gathering, the term spell has three meanings; one technical, one casual, and one flavorful.

• Technically, a spell is an object on the stack that exists before the spell takes effect. Players turn cards into spells by casting them, usually from non-land cards in their hand.
• In casual terms, "spell" is often used by players to refer specifically to non-permanent spell types, namely instants and sorceries.
• In the lore of the game, a spell can be almost any kind of magical effect requiring the expenditure of effort and energy by a magic user, usually based on some kind of ritual or knowledge.

Technical

A spell is either a card that has been cast and thus placed on the stack, or a copy of another spell. A card is only a spell when it is on the stack; in most other zones it is simply a card, or a permanent when on the battlefield. All card types other than lands are types of spells; even permanent cards are normally cast as spells before becoming permanents. Spells exist as game objects, and their rules determine what interactions and effects are possible between the casting of the spell and the spell taking effect by "resolving".

From the glossary of the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

Spell

A card on the stack. Also a copy (of either a card or another spell) on the stack. See rule 112, “Spells.”

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 112. Spells
  • 112.1. A spell is a card on the stack. As the first step of being cast (see rule 601, “Casting Spells”), the card becomes a spell and is moved to the top of the stack from the zone it was in, which is usually its owner’s hand. (See rule 405, “Stack.”) A spell remains on the stack as a spell until it resolves (see rule 608, “Resolving Spells and Abilities”), is countered (see rule 701.5), or otherwise leaves the stack. For more information, see section 6, “Spells, Abilities, and Effects.”
    • 112.1a A copy of a spell is also a spell, even if it has no card associated with it. See rule 707.10.
    • 112.1b Some effects allow a player to cast a copy of a card; if the player does, that copy is a spell as well. See rule 707.12.
  • 112.2. A spell’s owner is the same as the owner of the card that represents it, unless it’s a copy. In that case, the owner of the spell is the player under whose control it was put on the stack. A spell’s controller is, by default, the player who put it on the stack. Every spell has a controller.
    • 112.2a Some effects instruct a player to create a copy of a card and say they may cast it. In that case, the owner of that copy is the player who is instructed to create it and given permission to cast it.
  • 112.3. A noncopy spell’s characteristics are the same as those printed on its card, as modified by any continuous effects. See rule 613, “Interaction of Continuous Effects.”
  • 112.4. If an effect of a resolving spell or ability changes any characteristics of a permanent spell, the effect continues to apply to the permanent when the spell resolves. See rule 400.7.

    Example: If an effect changes a black creature spell to white, the creature is white when it enters the battlefield and remains white for the duration of the effect changing its color.

Spell ability

Any text on an instant or sorcery spell is a spell ability unless it's an activated ability, a triggered ability, or a static ability.

From the glossary of the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

Spell Ability

A kind of ability. Spell abilities are abilities that are followed as instructions while an instant or sorcery spell is resolving. See rule 113.3a.

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 113.3a Spell abilities are abilities that are followed as instructions while an instant or sorcery spell is resolving. Any text on an instant or sorcery spell is a spell ability unless it’s an activated ability, a triggered ability, or a static ability that fits the criteria described in rule 113.6.

Casual

Frequently, non-permanents are collectively referred to as "spells" in informal discussions. This includes sorceries and instants, along with the obsoleted mana sources and interrupts. This doesn't line up exactly with the game rules, which state that even permanent cards (except lands) are spells from the time they are cast until they resolve. This usage is instead based on the fact that typically, non-permanents only have gameplay effects while they're on the stack as spells, while permanents are relevant only after they enter the battlefield (and are no longer spells). Thus, whenever instants and sorceries are collectively referred to as a set theme or mechanic, it falls under the umbrella of "spells matter".

The original card type names "Summon" (instead of Creature) and "Enchant [type]" (instead of Enchantment - Aura) put more emphasis on the idea that the cards represented spells which resulted in permanents, rather than being permanents inherently. The modern type names create a clearer identity as permanents, which makes the casual terms "spells vs permanents" more distinct.

In some other contexts such as deckbuilding, spells can be referred to in the context of what they contrast to. "Spells vs. lands," asks about the land count ratio; "creature vs. noncreature spells," informs to what lengths the deck wishes to play to the combat step; "subtyped/tribal spells vs. those spells not subtyped", asks functionally what proportion of the deck satisfies a certain tribal playstyle.

Flavor

In the storyline, spells are the effect of a ritual that involves mental activity and a varying amount of both physical activity and the channeling of mana.

Mages use many different practices to channel mana, including chants, screams, and ritual motion. The concept of magic, unlike physics, has no unbreakable laws; spellcasters can use their spell do virtually anything.