Expressionism is a style in visual arts from the early 20th century, which is focused on expressing one's emotions, like angst, despair, anger and grief.

In music, expressionism is a mainly German/Austrian affair, closely linked to the second Viennese school (Arnold Schönberg, Alban Berg and Anton Webern), and atonality. Stylistically, it's the follow-up of late-romantic music. Emotions are taken to the extreme, leading to disturbing, unsettling and sometimes violent music.

Prime examples of expressionism are:
Richard Strauss: Elektra (1908)
Arnold Schönberg: Fünf Orchesterstücke, Op. 16 (1909), Erwartung, Op. 17 (1909), Pierrot Lunaire, Op. 21 (1912)
Béla Bartók: The miraculous mandarin, Op. 19 (1919)
Matthijs Vermeulen: Symphony No. 2 'prélude à la nouvelle journée' (1920)
Kurt Weill: Symphony No. 1 (1921)
Alban Berg: Wozzeck, Op. 7 (1921), Violin Concerto (1935)
Sergei Prokofiev: Symphony No. 3 in C minor, Op. 44 (1928)