A misnomer is a name or term that doesn't accurately describe what it's naming.

When something is called a misnomer, the name is misleading or technically incorrect, even though people might use it anyway. For example, peanuts are a misnomer because they're not actually nuts at all: they're legumes, like beans and peas, that grow underground. But we still call them peanuts because that's the name everyone knows.

The Hundred Years' War is another famous misnomer because it actually lasted 116 years. Koala bears aren't bears (they're marsupials), and jellyfish aren't fish (they're invertebrates). Sometimes these incorrect names stick around for so long that they become the standard way to refer to something, even though they're technically wrong.

You might hear someone say “That's a misnomer” when correcting an inaccurate name. If your friend calls the school library the “quiet room” but it's actually pretty noisy, you could point out that “quiet room” is a bit of a misnomer. This word helps us recognize when labels don't match reality.