What's the word meaning the use of the wrong word?

Solution 1:

One form of wrong-word-use is malapropism

the unintentional misuse of a word by confusion with one of similar sound, esp when creating a ridiculous effect, as in I am not under the affluence of alcohol

Another is misnomer

an incorrect or unsuitable name or term for a person or thing

And there is always that broader explanation (not limited to simple word substitution) by politicians, that they misspoke

to speak incorrectly, improperly, or misleadingly

Solution 2:

A little uncommon, but it's really specific for what you need: catachresis:

A misuse of a word; an application of a term to something which it does not properly denote.