122.1. A counter is a marker placed on an object or player that modifies its characteristics and/or interacts with a rule, ability, or effect. Counters are not objects and have no characteristics. Notably, a counter is not a token, and a token is not a counter. Counters with the same name or description are interchangeable.
122.1a A +X/+Y counter on a creature or on a creature card in a zone other than the battlefield, where X and Y are numbers, adds X to that object’s power and Y to that object’s toughness. Similarly, -X/-Y counters subtract from power and toughness. See rule 613.4c.
122.1b A keyword counter on a permanent or on a card in a zone other than the battlefield causes that object to gain that keyword. The keywords that a keyword counter can be are flying, first strike, double strike, deathtouch, haste, hexproof, indestructible, lifelink, menace, reach, trample, and vigilance, as well as any variants of those keywords. See rule 613.1f.
122.1c The number of loyalty counters on a planeswalker on the battlefield indicates how much loyalty it has. A planeswalker with 0 loyalty is put into its owner’s graveyard as a state-based action. See rule 704.
122.1d If a player has ten or more poison counters, that player loses the game as a state-based action. See rule 704. A player is “poisoned” if they have one or more poison counters. (See rule 810 for additional rules for Two-Headed Giant games.)
122.2. Counters on an object are not retained if that object moves from one zone to another. The counters are not “removed”; they simply cease to exist. See rule 400.7.
122.3. If a permanent has both a +1/+1 counter and a -1/-1 counter on it, N +1/+1 and N -1/-1 counters are removed from it as a state-based action, where N is the smaller of the number of +1/+1 and -1/-1 counters on it. See rule 704.
122.4. If a permanent with an ability that says it can’t have more than N counters of a certain kind on it has more than N counters of that kind on it, all but N of those counters are removed from it as a state-based action. See rule 704.
122.5. If an effect says to “move” a counter, it means to remove that counter from the object it’s currently on and put it onto a second object. If either of these actions isn’t possible, it’s not possible to move a counter, and no counter is removed from or put onto anything. This may occur if the first and second objects are the same object; if the first object doesn’t have the appropriate kind of counter on it; if the second object can’t have counters put onto it; or if either object is no longer in the correct zone.
122.6. Some spells and abilities refer to counters being put on an object. This refers to putting counters on that object while it’s on the battlefield and also to an object that’s given counters as it enters the battlefield.
122.6a If an object enters the battlefield with counters on it, the effect causing the object to be given counters may specify which player puts those counters on it. If the effect doesn’t specify a player, the object’s controller puts those counters on it.
122.7. An ability that triggers “When/Whenever the Nth [kind] counter” is put on an object triggers when one or more counters of the appropriate kind are put on the object such that the object had fewer than N counters on it before the counters were put on it and N or more counters on it after.
122.8. If a triggered ability instructs a player to put one object’s counters on another object and that ability’s trigger condition or effect checks that the object with those counters left the battlefield, the player doesn’t move counters from one object to the other. Rather, the player puts the same number of each kind of counter the first object had onto the second object.