auxiliary character (in kanbun)
Technical term in the study of kanbun (classical Chinese texts read in Japanese). Refers to grammatical particles or function characters that mark relationships between words, such as 而, 於, 于, 之, etc.
In kanbun, auxiliary characters like 而 and 於 indicate the structure of the sentence.
From 助 (じょ, 'help, assist') + 字 (じ, 'character'). The term is a direct calque from Chinese 助字 (zhùzì), used in traditional Chinese philology for function words.