Runny nose (also called rhinorrhea) is excessive discharge of clear, watery, thin mucus from the nasal passages, often dripping out the front of the nose or down the back of the throat (postnasal drip). It results from increased mucus production by the nasal mucosa, typically triggered by viral upper respiratory infections (common cold), allergies (allergic rhinitis from pollen, dust, pet dander, or mold), irritants (smoke, strong odors, pollution, dry air, or temperature changes), sinusitis, non-allergic rhinitis (e.g., vasomotor rhinitis from weather shifts or spicy foods), or less commonly nasal polyps, foreign bodies (especially in children), cerebrospinal fluid leak (clear, one-sided, salty-tasting discharge after head injury), or certain medications. The discharge is usually clear and watery in viral/allergic cases but can become thicker, yellow/green, or purulent if a secondary bacterial infection develops; accompanying symptoms often include nasal congestion, sneezing, itchy nose/eyes/throat, watery eyes, sore throat from drip, or cough. It is very common, especially during cold/flu season or allergy peaks, and tends to be self-limiting in viral cases but persistent in chronic allergic or irritant-related forms.