noun
supplement; top-up; making up a deficiency
Used for something that fills a gap or makes up for a shortfall, often in the phrase 足しになる (to be helpful as a supplement).
See also: 足しになる
少しでも足しになればと思って、お金を渡した。
I gave him some money, hoping it would help even a little.
この収入は家計の足しになる。
This income helps supplement the household budget.
補足 is a more formal term for supplementing or adding to something, often used in academic or official contexts, while 足し is a casual, everyday word for making up a shortfall.
Derived from the verb 足す (たす, 'to add'), nominalized as 足し. The kanji 足 carries the meaning of 'sufficiency' or 'adding'.