Legal Definition
In a legal context, joinder refers to the formal process of bringing or adding parties to a lawsuit or legal action, often in response to an existing claim or suit. It is the mechanism by which a court determines which parties are properly included in a dispute and who should be legally involved in the litigation.
Plain-English Translation
Imagine you have a big problem (a lawsuit), and now you need to decide exactly who gets to be part of that problem. Joinder is the rule that says how to officially add new people or claims to the existing group of people who are fighting in court.