'Devour' was introduced in the Shards of Alara Block and it currently appears on a number of cards including Gorger Wurm and Mycoloth

702.79. Devour

• 702.79a. Devour is a static ability. "Devour N" means "As this object enters the battlefield, you may sacrifice any number of creatures. This permanent enters the battlefield with N +1/+1 counters on it for each creature sacrificed this way."
• 702.79b. Some objects have abilities that refer to the number of creatures the permanent devoured. "It devoured" means "sacrificed as a result of its devour ability as it entered the battlefield."

Translation:

• 702.79a. Devour is always active, it doesn't trigger or respond to anything, but when it comes into play you may sacrifice creatures to it. The card enters play with +1/+1 counters equal to the number of creatures sacrificed to it.
• 702.79b. If it refers to the number of creatures 'devoured' it means the number of creatures sacrificed to it.