A "mucous cyst of the oral mucosa" (also known as a "mucocele") is a clinical term that refers to two related phenomena: mucus extravasation phenomenon, and mucus retention cyst.
The former is a swelling of connective tissue consisting of collected mucin due to a ruptured salivary gland duct usually caused by local trauma, in the case of mucus extravasation phenomenon, and an obstructed or ruptured salivary duct (Parotid duct) in the case of a mucus retention cyst. Inflammation of sinuses can also cause this condition.
The mucocele has a bluish translucent color, and is more commonly found in children and young adults.