The Briar Patch is a region of Space on the edge of the Federation. it represents an obstacle to Federation Expansion in the area.
Place Holder for Richard to clarify
No mentions, but assumed to exist.
The Briar Patch is a gasseous cloud several hundred light years in diameter. It is quite an investment of time for starships to go around it. Weeks at least.
The Briar Patch also holds several pulsars, neutron stars and other stellar phenomena that make it a treacherous territory to try and traverse.
The Briar Patch is home to weird energy sources, currents and flows. Not all of these are well understood. They pose a positive threat to any starship entering the Briar Patch itself.
The Briar Patch is also a factor in regional politics, used as a barrier, a wall, leverage and in other ways by local nations, and less well organized groups.
Because the Briar Patch interferes with sensors, it's some times used by pirates and renegades. On the street the pirates who use the briar patch to hide are colloquially called "Insane" and "Dangerous Lunatics". No one knows how many such ships have already been eaten by the effects of the Briar Patch.
It is suspected that the Briar Patch is the source of the Smelly Jelly entities.