Definitions for orestesɔˈrɛs tiz, oʊˈrɛs-
Random House Webster's College Dictionary
O•res•tesɔˈrɛs tiz, oʊˈrɛs-(n.)
the legendary son of Agamemnon and Clytemnestra, who, together with his sister Electra, avenged the murder of his father by killing his mother and her lover, Aegisthus.
(Greek mythology) the son of Agamemnon and Clytemnestra; his sister Electra persuaded him to avenge Agamemnon's death by killing Clytemnestra and Aegisthus
The Nuttall Encyclopedia
the son of Agamemnon and Clytemnestra, and brother of Electra and Iphigenia, who killed his mother to avenge the murder by her of his father and went mad afterwards, but was acquitted by the Areopagus and became king of Argos and Lacedæmon; his friendship for Pylades, who married his sister Electra, has passed into a proverb; the tragic story is a favourite theme of the Greek tragedians.