Equip [cost] ((Cost): Attach to target creature you control. Equip only as a sorcery.)|
1% Artifact 96% Multicolor 3%
Equip is a keyword ability found on artifacts with the subtype "equipment". By paying the equip cost, you can attach the artifact to a creature, who is referred to in the text as the "equipped creature".
Equipping is played as a sorcery. When an equipped creature leaves play, unlike auras, the equipment remains in play.
The first cards with equip appeared in Mirrodin. It was so flavorful and so popular, that it went evergreen instantaneously. Ninth Edition was the first core set to feature the keyword. 
From the Comprehensive Rules (Commander 2018 (August 10, 2018))
- 702.6. Equip
- 702.6a Equip is an activated ability of Equipment cards. “Equip [cost]” means “[Cost]: Attach this permanent to target creature you control. Activate this ability only any time you could cast a sorcery.”
- 702.6b For more information about Equipment, see rule 301, “Artifacts.”
- 702.6c “Equip [quality] creature” is a variant of the equip ability. “Equip [quality] [cost]” means “[Cost]: Attach this permanent to target [quality] creature you control. Activate this ability only any time you could cast a sorcery.” This ability doesn’t restrict what the Equipment may be attached to.
- 702.6d An “equip [quality] creature” ability is an equip ability, and an “equip [quality] creature” cost is an equip cost. Any effect that modifies how or whether a player may activate an equip ability of an object affects “equip [quality] creature” abilities of that object. Any effect that increases or reduces an equip cost will increase or reduce a “equip [quality] creature” cost.
- 702.6e If a permanent has multiple equip abilities, any of its equip abilities may be activated.
From the glossary of the Comprehensive Rules (Commander 2018 (August 10, 2018))
- A keyword ability that lets a player attach an Equipment to a creature they control. See rule 301, “Artifacts,” and rule 702.6, “Equip.”
- Whispersilk Cloak -
Artifact — Equipment
Equipped creature is unblockable.
Equipped creature has shroud. (It can't be the target of spells or abilities.)
Equip (: Attach to target creature you control. Equip only as a sorcery.)