Expressionism is a modernist movement in drama and theatre that developed in Europe (principally Germany) in the early decades of the 20th century and later in the United States. It forms part of the broader movement of Expressionism in the arts.
There was a concentrated Expressionist movement in early 20th century German theatre of which Georg Kaiser and Ernst Toller were the most famous playwrights. Other notable Expressionist dramatists included Reinhard Sorge, Walter Hasenclever, Hans Henny Jahnn, and Arnolt Bronnen. They looked back to Swedish playwright August Strindberg and German actor and dramatist Frank Wedekind as precursors of their dramaturgical experiments.
Australian Catholic University Excellent lecture notes on Strindberg and Expressionism.
Learn.co.uk Useful student guide to Expressionism in theatre.