Use of several keys at once, a practice common in the expanded tonalities of the 20th century. You will have your strongest sense of bitonality and polytonality in the works of Stravinsky and Bartók: here it is common for one melodic line or group of instruments to be in one key, while others go on in other keys.

music that uses two or more different keys at the same time

the use of two different keys simultaneously. Despite much loose talk, true polytonality is rare. Upper structures (q.v.) and outside playing do not usually qualify because there is always a strong single underlying tonality.