Misnomer

[ mis-noh-mer ]

noun

1. a wrong or inaccurate name or designation.
2. a wrong or inaccurate use of a name or term.

Examples of misnomer in a sentence:

• Since Fred is very skinny, calling him Fat Fred is indeed a misnomer.  
• Labeling the product as low-fat is a bit of a misnomer because the item’s actual fat content is really quite high.