Prevalence means how common or widespread something is in a particular group or place. When doctors study the prevalence of flu in a school, they're measuring how many students have it compared to the total number of students. If 30 out of 100 students have the flu, that's a high prevalence.

You'll often hear this word in health and science contexts. Researchers might study the prevalence of nearsightedness among children, or the prevalence of oak trees in a forest. The word helps us understand how much something exists relative to everything else, giving us a sense of its frequency or proportion.

Think of prevalence as a snapshot of how common something is at a particular time. If your teacher notices the prevalence of tardiness in your class, she's observing that being late has become a frequent problem. If a librarian tracks the prevalence of mystery books being checked out, she's noticing they're especially popular compared to other genres.