What is meant by revenge tragedy?

In The Duchess of Malfi, Webster uses many of the techniques of the revenge play--a form that was starting to fall out of fashion at the time of the play’s first staging--but for different and usually more complex purpose. The structure of the play, though simpler than that used in Webster’s The White Devil, is still fairly unorthodox. For example, the tragic and climactic scene of the heroine’s death occurs in the fourth act, leaving an entire act to follow. Many consider Webster’s vivid and masterful use of language to be the play’s greatest strength.

Many characters die in this play. The Duchess, her maid, her husband, and two of her children die as a result of Bosola’s orders. Subsequently, a servant, the Cardinal, and Ferdinand are killed out of revenge for the deaths of the Duchess, Antonio, and Julia. To top it off, Bosola dies from stab wounds from wild Ferdinand. Before he dies, however, Bosola recounts the murders for those who remain and accepts death after seeking justice.