Mucus vs. Mucous: What’s the Difference?

The human body produces many fluids. Some of these fluids are mundane and some inspire disgust, but they are all necessary for the body’s daily operation and maintenance.

One such fluid is found in many membranes in various places throughout the body, but it is most commonly seen dripping from or being sneezed out of someone’s nose.

We have a word for this substance, but how should we spell it? Is mucous or mucus the correct spelling?

What is the Difference Between Mucus and Mucous?

In this article, I will compare mucus vs. mucous. I’ll use each word in a sentence, and, at the end, I’ll show you a helpful trick to use to tell them apart.

When to Use Mucus

Define mucus and define mucousWhat does mucus mean? Mucus is defined as the fluid secreted from mucous membranes. The word never functions as an adjective, only a noun. Here are some examples of the proper usage of mucus:

  • Because Callie did not cover her mouth when she sneezed, mucus flew into Shaw’s hair.
  • Jeff sobbed so hard that mucus oozed down his face and dripped onto his shirt.
  • Air pollution causes inflammation that creates mucus and swelling that blocks internal air passageways, he said. –LA Daily News

In certain medical contexts, a ball of mucus might be called a mucous ball. In this case, even though the ball itself is made up of mucus, mucous–not mucus–takes the function of an adjective. Mucous describes an attribute of the ball, therefore, a mucous ball and a mucous membrane.

When to Use Mucous

Definition of mucus definition What does mucous mean? Mucous is an adjective and it is defined as of or related to membranes which secrete the fluid we call mucus.

See the following sentences for examples.

  • Dani had an infection in all her mucous membranes.
  • As Jeff became more ill, a mucous film covered his airways and made it impossible to breath.
  • Pavlick said 99% of melanomas begin on the skin, with the other 1% starting in mucous membranes or, rarely, the eyes. –USA Today

Mucous is never a noun—it is only an adjective which describes membranes, not the fluid they secrete. English has a different word for the fluid itself, i.e., mucus.

Trick to Remember the Difference

Definition of mucous versus mucus Not sure you can keep track of mucous vs. mucus? Here is a helpful trick that can help you remember the difference.

Mucus is only ever a noun. Mucous is only an adjective. Mucus is secreted by mucous membranes.

You can remember to use mucous as an adjective since it ends in –ous. It shares this ending with many other adjectives, like disastrous, monogamous, and ferrous.


Is it mucus or mucous? Mucus and mucous are so similar in spelling, and pronunciation that people sometimes mix them up.

Mucous is an adjective. It describes certain membranes in the body. For example, a mucous membrane.

Mucus is a noun. It refers to the fluid secreted by these membranes.

Mucous is only an adjective, and mucus is only a noun. Mucous has the traditional -ous ending shared by many adjectives. Armed with this fact, you shouldn’t have trouble choosing mucus or mucous in your writing.