A Synovial Cyst is very similar to a Ganglion one often sees on the the back of one's wrist.  It is a benign (non-cancerous) problem.  As can be seen in the above illustration, a synovial facet cyst compresses the nerve root from above in stead of from below (like the herniated disk also seen on the other side).  Unlike the bulging disk, Synovial Facet Cysts do not generally resolve, and  usually require surgery.