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
