A keyword ability is a word or words that substitutes for a piece of rules text. Many keywords are summarized in reminder text, especially in core sets.

Keyword abilities are somewhat different from keyword actions, and both are distinct from ability words. Collectively, they are described as mechanics.

Ikoria: Lair of Behemoths introduced keyword counters.

From the glossary of the Comprehensive Rules (August 7, 2020—Double Masters) Keyword Ability A game term, such as “flying” or “haste,” used as shorthand for a longer ability or group of abilities. See rule 702, “Keyword Abilities.”

From the Comprehensive Rules (August 7, 2020—Double Masters) 702.1. Most abilities describe exactly what they do in the card’s rules text. Some, though, are very common or would require too much space to define on the card. In these cases, the object lists only the name of the ability as a “keyword”; sometimes reminder text summarizes the game rule. 702.1a If an effect refers to a “[keyword ability] cost,” it refers only to the variable costs for that keyword. Example: Varolz, the Scar-Striped has an ability that says “Each creature card in your graveyard has scavenge. The scavenge cost is equal to its mana cost.” A creature card’s scavenge cost is an amount of mana equal to its mana cost, and the activation cost of the scavenge ability is that amount of mana plus “Exile this card from your graveyard.” 702.1b An effect that grants an object a keyword ability may define a variable in that ability based on characteristics of that object or other information about the game state. For these abilities, the value of that variable is constantly reevaluated. Example: Volcano Hellion has the ability “Volcano Hellion has echo {X}, where X is your life total.” If your life total is 10 when Volcano Hellion’s echo ability triggers but 5 when it resolves, the echo cost to pay is {5}. Example: Fire//Ice is a split card whose halves have the mana costs {1}{R} and {1}{U}. Past in Flames reads “Each instant and sorcery card in your graveyard gains flashback until end of turn. The flashback cost is equal to its mana cost.” Fire//Ice has “Flashback {2}{U}{R}” while it is in your graveyard, but if you choose to cast Fire, the resulting spell has “Flashback {1}{R}.” 702.1c An effect may state that “the same is true for” a list of keyword abilities or similar. If one of those keyword abilities has variants or variables and the effect grants that keyword or counters of that keyword to one or more objects and/or players, it grants each appropriate variant and variable of that keyword. Example: Concerted Effort is an enchantment that reads “At the beginning of each upkeep, creatures you control gain flying until end of turn if a creature you control has flying. The same is true for fear, first strike, double strike, landwalk, protection, trample, and vigilance.” As that triggered ability resolves, each landwalk and protection ability from among creatures you control is granted to each creature you control. 702.1d An effect may refer to an object “with [keyword ability]” or “that has [keyword ability].” This means the same thing as an object “with a [keyword ability] ability” or an object “that has a [keyword ability] ability.”

Most abilities describe exactly what they do in the card’s rules text. Some, though, are very common or would require too much space to define on the card. In these cases, the object lists only the name of the ability as a “keyword”; sometimes reminder text summarizes the game rule.