Definition from Nolo’s Plain-English Law Dictionary
A person or entity who has an interest in the subject matter of a lawsuit and, therefore, can join in the lawsuit or may be brought into the suit by one of the parties to the legal action. A proper party is distinguished from a "necessary party" who the court will order joined (brought into) the suit. Example: Marianne and Issac both use the road on Allen's property to reach their vacation cabins. Marianne brings a lawsuit to establish the right to use the road. Issac could join the lawsuit as a proper party.