Someone who acts as a surrogate takes the place of another person. In the middle of a big Hollywood awards ceremony, if a celebrity has to go to the bathroom, a surrogate will take his or her place and fill the seat.
Surrogate comes from the Latin word surrogare, which means "to substitute." An uncle of aunt might refer to one of a niece or nephew as a surrogate child if they are very close, especially if they don't have biological children. Today, surrogate is often used to describe a woman who carries a baby for another couple.
