adjunct
/ˈa-ˌjəŋ(k)t/
noun
something joined or added to another thing but not essentially a part of it
a word or word group that qualifies or completes the meaning of another word or other words and is not itself a main structural element in its sentence
an adverb or adverbial phrase (such as heartily in "They ate heartily" or at noon in "We left at noon") attached to the verb of a clause especially to express a relation of time, place, frequency, degree, or manner
The website is designed as an adjunct to the book.
adjective
added or joined as an accompanying object or circumstance
attached in a subordinate or temporary capacity to a staff