Indeclinable

a. [Prefix in, not, and declinable; Fr. indéclinable, Sp. indeclinable, It. indeclinabile, Lat. indeclinabilis.] (Gram.) Not declinable; not varied by terminations; as, pondo, in Latin, is an indeclinable noun.

n. (Gram.) A word that is not declinable.