What is Joinder?

Legal Definition
In law, a joinder is the joining of two or more legal issues together. Procedurally, a joinder allows multiple issues to be heard in one hearing or trial and is done when the issues or parties involved overlap sufficiently to make the process more efficient or more fair. It helps courts avoid hearing the same facts multiple times or seeing the same parties return to court separately for each of their legal disputes. The term is also used in the realm of contracts to describe the joining of new parties to an existing agreement.
-- Wikipedia
Legal Definition
Joining or coupling together; uniting two or more constituents or elements in one; uniting with another person in some legal step or proceeding.
-- Black's Law Dictionary
Legal Definition
When the claims of parties in one lawsuit are joined.
Legal Definition
To act jointly with; to join.
-- Ballentine's Law Dictionary