A piano trio is a group of three instruments including a piano. Usually it is a piano, a violin and a cello. This combination of instruments has been a very popular form of chamber music from the Classical music period onwards. A piano trio can also mean a piece of music written for a piano trio to play.
Other combinations of piano with two instruments are often called by the featuring wind instrument. For example: Mozart wrote a trio for piano, clarinet and viola which is usually called a “clarinet trio”. Brahms wrote a trio for piano, violin and French horn which is usually called a “horn trio”.
Music for three people to play at one piano is called music for piano six hands.