Branchial cyst: Also called a branchial cleft cyst, this is a cavity that is a remnant from embryologic development present at birth in one side of the neck just in front of the large angulated muscle on either side (the sternocleidomastoid muscle). The cyst may not be recognized until adolescence as it enlarges its oval shape. It may develop a sinus or drainage pathway to the surface of the skin from which mucus can be expressed. Total surgical excision is the treatment of choice. Recurrence is not expected.