The duct of the parotid gland through which saliva produced is emptied into the buccal cavity.
It begins in the anterior portion of the parotid gland and passes anteriorly across the lateral surface of the masseter muscle, curving medially around the anterior margin of the muscle. In its course on the face it is in close relation to the buccal branch of the facial nerve and is slightly inferior to the transverse facial artery. It passes through the buccal fat pad and then penetrates the buccinator muscle and the mucosa of the mouth to end in a papilla opposite the second maxillary molar tooth.
It was discovered by Nicholas Steno (Stensen) and is often called Stensen's duct.
Source: http://www.uthscsa.edu/anatomy/Glossary.html