This question recently came up at lessons. "What is pocket?"
My definition: Pocket is the skill to synchronize with the bassist in such a way that the resulting beat/groove fills the dance floor with smiling faces.
It has many prerequisites.
1. Basic drum technique and coordination. Super chops, though helpful, are not necessarily required.
2. An equally skilled bassist. Pocket is a team effort.
3. Sophisticated aural skills: If you can't hear it in the playing of others you are unlikely to be able to hear it or create it in your own playing.
4. A desire to please others. "Drum sport", another branch of drumming, is more concerned with flash and visual magic over Pocket. In short pleasing oneself and/or showing off. Pocket is about inspiring movement and joy in a listener, in short pleasing others.
David Story, drummer, pianist, qualified online music teacher