replacing someone else or used instead of something else

Because she had no children of her own, her friend's son became a kind of surrogate child to her. 因为她自己没有孩子，所以她就把朋友的儿子当成了自己的孩子。

something that replaces or is used instead of something else

For some people, reading travel books is a surrogate for actual travel. 对某些人来说，读游记可以替代真正的旅行。

a person who acts or speaks in support of someone else, or does his or her job for a certain time

Both candidates in the election have turned to celebrity surrogates to excite the crowds. 参选的两位候选人都求助于名人代言人来激发观众的热情。

a woman who has a baby for another person who is unable to become pregnant or have a baby

She has agreed to act as a surrogate mother for her sister. 她已经同意为她姐姐代孕了。