Correct spelling, explanation: the word apartment is usually tricky because its spelling is different from French and Italian equivalents (appartement, appartimento). This American word for flat however is spelled with one, not two, p’s, so appartment is incorrect. This is probably due to simplifying spelling because of no change in pronunciation.

Definition of apartment:
noun, a flat (American English), a set of rooms especially on one floor; in the UK a luxurious set of rooms
This apartment was very elegant but too expensive for us.
Jackie needs to change her apartment because she got a job in the other part of the city.

Incorrect spelling, explanation: although French and Italian words for flat are spelled with double p (appartement, appartimento), the English word isn’t, so the correct form is apartment. One p in apartment is probably connected with the fact, that double p wouldn’t change pronunciation (at least not drastically), while the spelling is simplified. Therefore appartment is an incorrect spelling.

Incorrect spelling

Incorrect spelling

Incorrect spelling