joinder
The joining of a litigant to a suit.
The act of joining; a putting together; conjunction.
A contract of eternal bond of love, Confirm'd by mutuall ioynder of your hands.
Where it comes from
Substantivisation of the Old French verb joindre (“to join”), from Latin iungō (“join”).
Adapted from the Wiktionary entry joinder (CC BY-SA 4.0).