Formal term for something that serves as evidence or proof, often in legal, academic, or logical contexts. Similar to 証拠 but more literary.
See also: 証拠
この文書は彼の主張の証左となる。
This document serves as evidence for his claim.
Rarely used to mean a person who gives evidence; more commonly refers to the testimony or statement itself. In modern Japanese, 証人 is the usual word for a witness as a person.
In old court records, it says the witness gave testimony.
A Sino-Japanese compound in which 証 means “evidence” and 左 has its older sense “to assist or support,” hence evidence that supports a claim.