1 (also eyewitness)a witness (to sth) a person who sees sth happen and who can tell other people about it later 目击者: There were two witnesses to the accident. 有两个人目睹那起事故。

2 a person who appears in a court of law to say what he/she has seen or what he/she knows about sb/sth （法庭上作证的）证人: a witness for the defence/prosecution 被告╱原告证人

3 a person who sees sb sign an official document and who then signs it himself/herself （文件签署的）见证人