A misnomer is an incorrect or misleading name for a thing. They are often idiomatic.

Examples

• Anti-Semitism is prejudice against Jews, not all Semites.
• Arabic numerals originated in India, not in the Arab world.
• Asteroids are small planets, not small stars as their name suggests.
• Chinese checkers is not Chinese (or even Asian) in origin.
• Columbus Circle in New York is square, not circular.
• Comic books are illustrated stories of any genre or tone, not necessarily funny.
• Doves are aggressive birds, yet they symbolize peace.
• Dry cleaning often uses fluid such as naphtha, making clothes wet.
• The fundamental theorem of algebra is really a theorem of analysis, not of algebra.
• Koala bears are herbivorous marsupials, unrelated to the carnivorous bear family.
• The "lead" in modern pencils is made of graphite and clay, not lead.
• The Liberal Democratic Party of Japan is not a liberal party.
• Peanuts are legumes, not nuts.

Compare with

• malapropism