Champion

Champion is a keyword ability that allows you to upgrade one permanent into another, representing a powerful creature taking the place of another to fight in its stead.

A permanent is “championed” by another permanent if the latter exiles the former as the direct result of a champion ability.

History
Champion was introduced in Lorwyn, in which it featured on a cycle of rare non-shapeshifter creatures and on three uncommon shapeshifters with changeling, in whose case "champion" appears as "champion a creature". Morningtide featured cards that let you champion one of two creature types, the player gets to choose which one.

Though it was not a keyword at the time, the ability of Wormfang Drake from Judgement has an identical effect to "champion a creature", predating Lorwyn by about 5 years.

Rules
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Rulings

 * A creature's own champion ability won't allow you to exile that creature.
 * If a creature with champion leaves the battlefield before its enters-the-battlefield ability resolves, its leaves-the-battlefield ability will do nothing. Then its enters-the-battlefield ability will resolve. Its controller may exile a permanent of the appropriate quality they controls. If the player does, that card won't come back. If the player doesn't, nothing happens (because the creature with champion isn't around to be sacrificed).
 * If a creature with champion loses the champion ability (due to Humble, for example) and then leaves the battlefield, champion's leaves-the-battlefield ability won't trigger. The exiled card remains in exile.
 * All Lorwyn cards with both changeling and champion have "champion a creature." The ability allows you to exile any other creature you control.
 * All other Lorwyn cards with champion have "champion a [creature type]." The ability doesn't limit you to creatures. For example, a card with "champion a Kithkin" allows you to exile a Kithkin enchantment.