Guardian Ad Litem Definition: A guardian appointed to direct litigation on behalf and in the interests of a person otherwise incapable of managing their affairs. Related Terms: Guardianship, Ad Litem, Style of Cause, Alieni juris A guardian appointed to assist an infant or other mentally incapable defendant or plaintiff, or any such incapacitated person, that may be a party in a legal action, and to direct that litigation in the best interests of the incapable person. Find you are constantly looking up definitions? Try our search provider (works in most modern browsers) If you find an error or omission in Duhaime's Legal Dictionary, or if you have legal term suggestion, we'd love to hear from you!