Ferocious

Ferocious is an ability word introduced in Khans of Tarkir, wherein it was the Temur Frontier clan mechanic. "Ferocious" is also used to indicate a similar mechanic that doesn't use the ability word itself.

Description
Ferocious triggers an advantage for the player if they control a creature with power 4 or greater.

History
After Khans of Tarkir, the mechanic was also featured in Fate Reforged and on one card in Commander 2019 (Shamanic Revelation).

As mechanic without ability word
The mechanic was brought it back on single cards in Shadows over Innistrad and Amonkhet and as mini-themes in Core Set 2019 (three cards), and in Ravnica Allegiance (four cards), but every time without the ability word. This is perhaps the most times that a returning and previously named mechanic appeared without getting a name.

Over the following year, the draft archetype seems to have taken on Ferocious effectively as the default:
 * War of the Spark brought Ferocious back on five cards. This time, it was a bit stronger and focused in two colors, with the multicolored creature (Rubblebelt Rioters) not explicitly Ferocious, but works especially well with Ferocious.
 * Theros Beyond Death had it as the RG theme as shown by the multicolored signpost Warden of the Chained, and was featured, still unnamed, on seven cards.
 * Core Set 2021 brought back a Ferocious card each from War and Theros alongside four other new cards. The signpost was Leafkin Avenger.

For sets without the explicit theme, Ikoria: Lair of Behemoths has one card with Ferocious and Kaldheim has two cards.

Rulings
All rulings obviously work identically if the power would be any other number than 4.
 * Some ferocious abilities that appear on instants and sorceries use the word "instead." These spells have an upgraded effect if you control a creature with power 4 or greater as they resolve. For these, you only get the upgraded effect, not both effects.
 * Ferocious abilities of instants and sorceries that don't use the word "instead" will provide an additional effect if you control a creature with power 4 or greater as they resolve.
 * If an ability says "target creature with power 4 or greater," it checks that creature's power twice: when the creature becomes the target, and when the ability resolves. Once the ability resolves, it will continue to apply to the affected creature no matter what its power may become later in the turn.
 * If an ability affects a number of creatures with power 4 or greater but doesn't target them, it checks the creatures' powers only once: when the ability resolves. Once the ability resolves, it will continue to apply to the affected creatures no matter what their powers may become later in the turn.
 * If an ability triggers when a creature with power 4 or greater comes into play, it checks that creature's power only once: when that creature comes into play. The trigger checks a creature's initial power upon being put in play, so it will take into account counters that it comes into play with and static abilities that may give it a continuous power boost once it's in play (such as the one on Glorious Anthem). After the creature is already in play, boosting its power with a spell (such as Giant Growth), activated ability, or triggered ability won't allow this ability to trigger; it's too late by then. Once the ability triggers, it will resolve no matter what the creature's power may become while the ability is on the stack.
 * Some cards have abilities that say "At the end of your turn, if you control a creature with power 4 or greater, [effect]." These abilities have "intervening 'if' clauses." That means (1) the ability won't trigger at all unless you control a creature with power 4 or greater as your end step begins, and (2) the ability will do nothing if you don't control a creature with power X or greater by the time it resolves. (It doesn't have to be the same creature as the one that allowed the ability to trigger.) Power-boosting effects that last "until end of turn" will still be in effect when this kind of ability triggers and resolves. An ability like this will trigger a maximum of once per turn, no matter how many applicable creatures you control.
 * Some cards check the power of creature cards that aren't in play. If a creature card you look at this way has "*" in its power, the ability that defines its power works in all zones.

5-power matters

 * Scryfall search:

"5-power matters" was a theme of Shards of Alara that was a precursor to Ferocious; the Naya shard had a number of cards that look for creatures with power 5 or greater. The theme was not received well in part due to the lack of keyword, which Ferocious later corrected.

"Power 5 or Greater" was featured as rules card 5 of 5 in the Shards of Alara set.