Mucous describes something that produces or relates to mucus, the slippery, slightly sticky substance your body makes to protect certain surfaces inside you. The mucous membranes in your nose and throat are constantly making mucus to trap dust, germs, and other particles before they can reach your lungs.

Notice the spelling: mucous (ending in -ous) is the adjective that describes the membrane or tissue that makes the stuff, while mucus (ending in -us) is the actual gooey substance itself. So you have mucous membranes that produce mucus, just like you have nervous tissue in your nervous system or fibrous material made of fiber.

When you have a cold, your mucous membranes go into overdrive, producing extra mucus to help flush out the virus. That's why your nose runs. These same protective mucous membranes line your stomach, helping protect it from its own powerful digestive acids. Without mucous membranes working constantly throughout your body, you'd be vulnerable to all sorts of infections and injuries from the inside.