Definition. Runny nose is excess drainage, which may range from a clear fluid to thick mucus, produced by the nasal and adjacent tissues and blood vessels in the nose. The drainage of runny nose may run out of your nose or down the back of your throat or both. The terms rhinorrhea and rhinitis are often used interchangeably to refer to a runny nose. Strictly speaking, rhinorrhea refers to a thin, relatively clear nasal discharge. Rhinitis refers to inflammation of the nasal tissues from a number of causes, which usually results in a runny nose.