I agree with mambo - many of these responses fall into that category of saying a bunch of words without actually saying anything. Groove, pocket, "selling it", etc. are all terms that people throw around when they want something different than what they're hearing, but can't put into words what exactly it is they want.
To me, "groove" is a player's ability to convey their specific musical message precisely. For instance, in the song Superstition by Stevie Wonder - the line grooves because it's conveying very specific information. The note lengths are all very specific - the "tightness" the band has comes from all the players listening to and agreeing on the specific articulations and phrasing of each passage.
The same can be said of the drummer/bass relationship - when people tell you to "lock in" with the drummer, is that helpful information? Were you purposely trying to play against the drummer prior to being told that? Of course not, it's somebody telling you to do something, without giving you any useful information.
<mini-rant>