Misnomer

A misnomer is a word or term that suggests a meaning that is known to be wrong. Misnomers often arise because the thing received its name long before its correct nature was known, or because the nature of an earlier form is no longer the norm. A misnomer may also be simply a word that is used incorrectly or misleadingly. "Misnomer" does not mean "misunderstanding" or "popular misconception", and many misnomers remain in legitimate use (that is, being a misnomer does not always make a name incorrect).

Some of the sources of misnomers are:

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Wikipedia