Chromatic
Chromatic harmony means harmony (chords) that use notes which do not belong to the key the music is in (they are not in the key signature). Although Bach in the 18th century used chromatic harmony, it was 19th century composers who used it more and more. Wagner wrote music which was very chromatic: there were lots of sharps and flats and it kept modulating to different key areas. The chord at the beginning of his opera Tristan and Isolde is so famous that it is known as the Tristan chord. It is very chromatic. The music is full of tension because it leaves us wondering which key the music is in.