adjunct
Linguistics term for an optional element in a clause that adds extra information, such as time, place, or manner, without being a core argument.
"Yesterday" functions as an adjunct in this sentence.
An adjunct is not an obligatory element of a sentence.
修飾語 (modifier) is a broader term that includes both adjuncts and other modifying elements; 付加詞 is specifically a syntactic adjunct in clause structure.
Compound of 付加 (fuka, 'addition') and 詞 (shi, 'word' or 'part of speech'), likely a calque from English 'adjunct' in linguistics.