A misnomer is a wrong or unsuitable name. It’s a misnomer to call your grandmother “Grandfather,” the same way it’s a misnomer to call a chair with four legs that doesn’t move unless you drag it across the floor, a rocking chair.
From the Middle-French misnomer “to misname” evolved our English misnomer, a noun indicating a lack of fit when it comes to naming. Synonyms include, predictably, misnamed, but also mistermed and miscalled. You’ll notice the repetition of the mis- at the front of all those words. It comes from the French mes-, which means “wrongly.”