Riftbound Buff

None of the 23 cards with Buff print a reminder explaining it, so there is no official wording to quote here. Rather than guess at the rule, the cards themselves are below.

23cards3median energyEUR 0.02cheapest

Which domains get it

Calm10

Body7

Chaos5

Order3

Fury2

Which sets print it

Unleashed · 13Radiance · 10

Cards that mention Buff

These do not have Buff themselves, their rules text refers to it.

Adaptatron

When I conquer, you may kill a gear. If you do, buff me. (If I don't have a buff, I get a +1 might buff.)

Albus Ferros

When you play me, spend any number of buffs. For each buff spent, channel 1 rune exhausted.

Aphelios - Exalted

When you attach an Equipment to me, choose one that hasn't been chosen this turn —Ready 2 runes.Channel 1 rune exhausted.Buff a friendly unit.

Aphelios - Exalted

When you attach an Equipment to me, choose one that hasn't been chosen this turn —Ready 2 runes.Channel 1 rune exhausted.Buff a friendly unit.

Aphelios - Exalted

When you attach an Equipment to me, choose one that hasn't been chosen this turn —Ready 2 runes.Channel 1 rune exhausted.Buff a friendly unit.

Arena Bar

exhaust: Buff an exhausted friendly unit. (If it doesn't have a buff, it gets a +1 might buff.)

Buhru Captain

When you play me, you may draw 1 or buff me. (To buff a unit, give it a +1 might buff if it doesn't already have one.)

Call to Glory

[Reaction] (Play any time, even before spells and abilities resolve.) As you play this, you may spend a buff as an additional cost. If you do, ignore this spell's cost. Give a unit +3 might this turn.

Cithria of Cloudfield

When you play another unit, buff me. (If I don't have a buff, I get a +1 might buff.)

Fae Dragon

When you play me, buff up to four friendly units. (Give each a +1 might buff if it doesn't have one.) When you spend a buff, play a Gold gear token exhausted.

Fae Dragon (Spiritforged Nexus Night Promo)

When you play me, buff up to four friendly units. (Give each a +1 might buff if it doesn't have one.) When you spend a buff, play a Gold gear token exhausted.

Karma - Channeler

[Vision] (When you play me, look at the top card of your Main Deck. You may recycle it.) When you recycle one or more cards, buff a friendly unit. (If it doesn't have a buff, it gets a +1 might buff. Runes aren't cards.)