Often used in the phrase "in the pocket" to describe someone playing a musical instrument in such a way that the groove is very solid and with a great feel. When a drummer keeps a good metronomic pulse (keeping time), and makes the groove feel really good, and maintains this feel for an extended period of time, never wavering, this is often referred to as a deep pocket.

For more information, including the history of this term and its use, read Bart Elliott's article entitled In The Pocket.