In polite circles and among health care professionals, they tend to call it mucus. Some call it phlegm. Others may simply call it snot.
No matter which word you use, when you have a cold or flu, you usually have an increased supply of it. At your sickest, your sinuses may rev up production almost as fast as you can clear the stuff away.
Still, doctors say, all that gunk in your nose really does have a role in the body and a useful purpose during a cold.
Mucus in the nose and sinuses forms a blanket or coating over the mucous membranes lining the upper airways, explained Dr. Thomas Pasic, an ear, nose and throat specialist at the University of Wisconsin at Madison.
"It's sticky, so it's constantly trapping miniscule particles from the air you breathe in, such as allergens, pollution and dust to filter it out before it can reach the lungs," Pasic said.
Pasic added that mucus is constantly being formed, and we typically make about a quart of it a day. But we often don't notice it because we usually swallow the secretion rather than honking it out of the nose.
According to Pasic, you notice it when you make more than a quart daily or it becomes thicker, because you start to sniff. "A cold or an allergy sets mucus production out of whack."
An increase in mucus and its thickness is the way your nose and sinuses respond to the presence of an irritant, whether it's an invading virus, bacteria, or allergen.
Inflammation in the nose and greater quantities of mucus can also be triggered by foods or things in the environment. These include cigarette smoke, chemical exposures, strong smells and perfumes, as well as substances found in milk, red wine and beer.
There's very little that is magical about mucus, which is mostly water and salt with proteins, such as antibodies, to fight off infection mixed in. Still, people often wonder whether its color or consistency has any significance on the length or severity of their cold.
"Color is most predictive of the length of symptoms," suggested Dr. Matthew Rank, an allergist at the Mayo Clinic in Rochester, Minn. "As the mucus becomes darker in color, it may mean that the immune system is beginning to clear the infection."
But Rank is quick to point out that when it comes to a cold, "color doesn't matter for making a clinical diagnosis" and it doesn't influence his treatment decisions.
Ear, nose and throat specialists suggest that the normal color of mucus is clear. A cloudy or white nasal mucus may signal a cold.
Large amounts of yellow or green mucus tend to be a sign of bacterial infection. Occasionally, mucus takes on a brownish hue when it's tinged with a little blood, or possibly from the nostrils of heavy smokers or people exposed to lots of pollutants.
Asked whether the color or consistency of mucus had any importance, Dr. William Marshall, an infectious disease specialist at the Mayo Clinic, said it's probably not of great significance.
"I wouldn't hang my hat on it," he said. "There's not much useful information there for patients."
Mucus keeps membranes moist, said Dr. Ralph Metson, a sinus surgeon at Massachusetts Eye and Ear Infirmary in Boston. It basically keeps the nose clean.