Definition from Nolo’s Plain-English Law Dictionary
A person appointed by a probate court to represent an estate during a lawsuit. (Ad litem is Latin for "during the litigation.") An administrator ad litem is appointed only if there is no existing executor or administrator of the estate, or if the executor or administrator has conflicting interests. For example, Jerry's will leaves most of his property to his brother, Jeff, and also names Jeff as executor of the will. But Jerry's sister, Janine, feels that Jerry made the will under improper pressure from Jeff and brings a lawsuit to challenge it. The court appoints an administrator ad litem to represent Jerry's estate while the lawsuit is in progress. Also known as administrator ad prosequendum, meaning administrator "during the prosecution."