An apartment is a private residence in a building or house that's divided into several separate dwellings. An apartment can be one small room or several.
An apartment is a flat — it's usually a few rooms that you rent in a building. Your apartment might be in a fancy high rise with a doorman and an elevator, or over your parents' garage. Since the 1640s, an apartment has meant "separate rooms within a house," from the Italian word appartimento, which literally means "a separated place."