witness作名词讲是可数名词，有目击者，证据等含义。可用作及物动词和不及物动词，有为...作证，证明等含义。

一.用作名词

1.目击者 固定词组 witness of

witnesses(at the scene) of the accident 事故(现场)的目击者

I was a witness to their quarrel.

我是他们吵架时的目击者。

2.证人

a defence/prosecution witness 被告的/原告的证人

a witness for the defence/prosecution 被告的/原告的证人

(注: witness for也不是词组,只是一般后面跟for)

3.证明,证据

give witness on behalf of an accused person 为被告方作证

His ragged clothes were (a) witness to his poverty. 他的破烂衣物可以证明他很贫穷。

二.用作动词

1.目击(当场见到某事物)

witness an accident, a murder, a quarrel

目击一事故、 谋杀事件、 一场争吵

We were witnessing the most important scientific development of the century.

我们亲眼见到本世纪最重要的科学进展。

2.为别人签署（文件）时作证（尤指自己也同时签字者）

witness the signing of a contract

在一份合同签署时作证