What is a subordinate word that is never inflected (a preposition, conjunction, interjection); or a word that can not be used except in compositions; as, ward in backward, ly in lovely called?

particle — (Gram.) A subordinate word that is never inflected (a preposition, conjunction, interjection); or a word that can not be used except in compositions; as, ward in backward, ly in lovely. (Webster's 1913.)

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