Mucus is the slippery, sticky substance your body produces to protect and clean the inside of your nose, throat, lungs, and other passageways. When you have a cold and blow your nose, that's mucus doing its job of trapping germs, dust, and other particles that shouldn't be in your body.

Your body makes mucus all the time, not just when you're sick. Normally you don't even notice it because it's thin and clear, quietly protecting the delicate tissues inside your respiratory system. But when you catch a cold or breathe in irritating dust, your body produces extra mucus to flush out the invaders. That's when it becomes thicker and more noticeable.

The word mucus refers to the substance itself (it's a noun), while mucous (spelled with an “o” at the end) describes something that produces or relates to mucus, like your mucous membranes. These membranes are the soft, moist tissues lining your nose and throat that make mucus in the first place.

Though mucus might seem gross, especially when you're sick, it's actually your body's clever defense system at work, catching harmful particles before they can reach your lungs and cause real problems.