The term inter partes is the Latin for "between the parties." It can be distinguished from in rem, referring to a legal action whose jurisdiction is based on the control of property, or ex parte referring to a legal action that is by a single party.
Lawsuits where all interested parties have been served with adequate notices and are given a reasonable opportunity to attend and to be heard are referred to as inter partes proceedings or hearings. When a judgment is given, subject to any right of appeal, it would be inconvenient if the same issues could be endlessly relitigated by the same parties, so they are all bound by the result. However, anyone who was not a party to those proceedings and who can demonstrate a legitimate interest in reopening the issue, is entitled to petition the court for the right to be heard. However, in some circumstances, the judgment is given in rem, i.e. it binds everyone whether they were a party to the case or not.
Contracts can also be said to be inter partes and various laws can be relied upon to create and vest rights which exist on an inter partes basis only, i.e. they do not attach as an attribute to a person's status and so become in rem rights.