[count] something that is joined or added to another thing but is not an essential part of it

usually + to

grammar :a word or phrase (such as an adverb or prepositional phrase) that provides added information about the meaning of a verb in a sentence by expressing a relation of time, place, manner, etc.

added or joined in order to be used with something

added to a teaching staff for only a short time or in a lower position than other staff