What is not declinable; not varied by inflective terminations; as, nihil (nothing), in Latin, is an indeclinable noun. -- n. An indeclinable word called?

indeclinable — Not declinable; not varied by inflective terminations; as, nihil (nothing), in Latin, is an indeclinable noun. -- n. An indeclinable word. (Webster's 1913.)

Sources