粘液 (nián yè) literally combines 粘 (nián), meaning 'sticky' or 'adhesive', and 液 (yè), meaning 'liquid' or 'fluid'. Together, they refer to mucus — a slippery, viscous substance secreted by mucous membranes in the human body, such as those lining the respiratory, digestive, and reproductive tracts. It serves protective functions: trapping dust and pathogens, lubricating tissues, and aiding digestion.

This term is used primarily in medical, biological, and health-related contexts — for example, when describing symptoms like excessive phlegm, nasal discharge, or gastrointestinal secretions. It is a formal, scientific word; everyday speech often uses more colloquial terms like 痰 (tán, 'phlegm') or 鼻涕 (bí tì, 'nasal mucus'). While 粘液 can appear in veterinary or zoological discussions (e.g., slug secretion), its dominant usage relates to human physiology and clinical health.