A runny nose is never any fun, especially when it comes with a cold or allergies. It can interrupt your daily life, making you reach for the tissue box every few minutes. But what is a runny nose exactly? A runny nose is a nasal discharge of mucus. Your doctor might use the term “rhinorrhea” to diagnose the runny nose. “Rhino” is a Greek prefix meaning “nose,” and “–rrhea” means “flow” or “discharge.”