What is Non-Joinder?

Legal Definition
Pleading, practice. The omission of some one of the persons who ought to have been made a plaintiff or defendant along with others is called a non joinder.

2. In actions upon contracts, where the contract has been made, with several, if their interest were joint, they miist all, if living, join in the action for its breach. 8 S., & R. 308; 10 S. & R. 257; Minor, 167; Hardin, 508. In such case the non joinder must be pleaded in abatement. Id.; 3 Bouv. Inst. n. 2749.
-- Bouviers Law Dictionary
Legal Definition
The omission to join some person as party to a suit, whether as plaintiff or defendant, who ought to have been so joined, according to the rules of pleading and practice.
-- Black's Law Dictionary
Legal Definition
The failure to include therein necessary parties to an action.
-- Ballentine's Law Dictionary