Tap
Tap is a keyword action that is usually represented on cards with 
symbol.
From the ()
Tap symbol
Originally cards used the word tap explicitly (e.g., the text on Samite Healer in Unlimited was "Tap to prevent 1 damage to any target.") or were of type "Mono Artifact" (Cyclopean Tomb). Revised introduced the tap symbol as a tilted T inside a gray circle. In 4th Edition, it was changed to a white curved arrow with a tilted black rectangle representing the tapping card. In 8th Edition, it was changed to a black curved arrow without the rectangle behind it .
From the ()
Actions that cause a card to tap
- Using its activated ability if that ability requires it to tap
- Declaring a creature as an attacker (see Declare Attackers Step)
- Regenerating