A penthouse is fancy apartment on the very highest floor of a tall building. The penthouse can be a status symbol. Just ask the characters from The Jeffersons — they moved on up to a “deluxe apartment in the sky.”
While you can use the word penthouse for any top floor apartment, it usually implies something luxurious. A penthouse is often the fanciest and most expensive apartment in the building. Sometimes a penthouse even has its own private elevator. The first penthouses appeared in the 1920s, some taking up entire top floors, though the word has been in use much longer. Originally, it meant "simple structure," from the Old French apentis, "attached building."