A target is the recipient of some spell or effect. A spell can require you to target a creature, for instance.
There are some spells that can redirect a target. Spell redirection (Change the target of target spell with a single target.) is primary in both blue and red.
From the Comprehensive Rules (Ravnica Allegiance (January 25, 2019))
- 114. Targets
- 114.1. Some spells and abilities require their controller to choose one or more targets for them. The targets are object(s) and/or player(s) the spell or ability will affect. These targets are declared as part of the process of putting the spell or ability on the stack. The targets can't be changed except by another spell or ability that explicitly says it can do so.
- 114.2. Only permanents are legal targets for spells and abilities, unless a spell or ability (a) specifies that it can target an object in another zone or a player, or (b) targets an object that can't exist on the battlefield, such as a spell or ability. See also rule 114.4.
- 114.3. The same target can't be chosen multiple times for any one instance of the word "target" on a spell or ability. If the spell or ability uses the word "target" in multiple places, the same object or player can be chosen once for each instance of the word "target" (as long as it fits the targeting criteria). This rule applies both when choosing targets for a spell or ability and when changing targets or choosing new targets for a spell or ability (see rule 114.7).
- 114.4. Some spells and abilities that refer to damage require "any target," "another target," "two targets," or similar rather than "target [something]." These targets may be creatures, players, or planeswalkers. Other game objects, such as noncreature artifacts or spells, can't be chosen.
- 114.5. A spell or ability on the stack is an illegal target for itself.
- 114.6. A spell or ability that requires targets may allow zero targets to be chosen. Such a spell or ability is still said to require targets, but that spell or ability is targeted only if one or more targets have been chosen for it.
- 114.7. Some effects allow a player to change the target(s) of a spell or ability, and other effects allow a player to choose new targets for a spell or ability.
- 114.7a If an effect allows a player to "change the target(s)" of a spell or ability, each target can be changed only to another legal target. If a target can't be changed to another legal target, the original target is unchanged, even if the original target is itself illegal by then. If all the targets aren't changed to other legal targets, none of them are changed.
- 114.7b If an effect allows a player to "change a target" of a spell or ability, the process described in rule 114.7a is followed, except that only one of those targets may be changed (rather than all of them or none of them).
- 114.7c If an effect allows a player to "change any targets" of a spell or ability, the process described in rule 114.7a is followed, except that any number of those targets may be changed (rather than all of them or none of them).
- 114.7d If an effect allows a player to "choose new targets" for a spell or ability, the player may leave any number of the targets unchanged, even if those targets would be illegal. If the player chooses to change some or all of the targets, the new targets must be legal and must not cause any unchanged targets to become illegal.
- 114.7e When changing targets or choosing new targets for a spell or ability, only the final set of targets is evaluated to determine whether the change is legal.
Example: Arc Trail is a sorcery that reads "Arc Trail deals 2 damage to any target and 1 damage to another target." The current targets of Arc Trail are Runeclaw Bear and Llanowar Elves, in that order. You cast Redirect, an instant that reads "You may choose new targets for target spell," targeting Arc Trail. You can change the first target to Llanowar Elves and change the second target to Runeclaw Bear.
- 114.8. Modal spells and abilities may have different targeting requirements for each mode. An effect that allows a player to change the target(s) of a modal spell or ability, or to choose new targets for a modal spell or ability, doesn't allow that player to change its mode. (See rule 700.2.)
- 114.9. Some objects check what another spell or ability is targeting. Depending on the wording, these may check the current state of the targets, the state of the targets at the time they were selected, or both.
- 114.9a An object that looks for a "[spell or ability] with a single target" checks the number of times any objects or players became the target of that spell or ability when it was put on the stack, not the number of its targets that are currently legal. If the same object or player became a target more than once, each of those instances is counted separately.
- 114.9b An object that looks for a "[spell or ability] that targets [something]" checks the current state of that spell or ability's targets. If an object it targets is still in the zone it's expected to be in or a player it targets is still in the game, that target's current information is used, even if it's not currently legal for that spell or ability. If an object it targets is no longer in the zone it's expected to be in or a player it targets is no longer in the game, that target is ignored; its last known information is not used.
- 114.9c An object that looks for a "[spell or ability] that targets only [something]" checks the number of different objects or players that became the target of that spell or ability when it was put on the stack (as modified by effects that changed those targets), not the number of those objects or players that are currently legal targets. If that number is one (even if the spell or ability targets that object or player multiple times), the current state of that spell or ability's target is checked as described in rule 114.9b.
- 114.10. Spells and abilities can affect objects and players they don't target. In general, those objects and players aren't chosen until the spell or ability resolves. See rule 608, "Resolving Spells and Abilities."
- 114.10a Just because an object or player is being affected by a spell or ability doesn't make that object or player a target of that spell or ability. Unless that object or player is identified by the word "target" in the text of that spell or ability, or the rule for that keyword ability, it's not a target.
- 114.10b In particular, the word "you" in an object's text doesn't indicate a target.
From the glossary of the Comprehensive Rules (Ravnica Allegiance (January 25, 2019))
- Any Target
- A spell or ability may require "any target." "Any target" is the same as "target creature, player, or planeswalker." See rule 114.4.