According to the Merriam-Webster Dictionary, the definition of the word 'surrogate' is:
1: one appointed to act in place of another / 2: one that serves as a substitute
So, as one can see, both definitions imply that a surrogate is a woman who acts as a gestational carrier, that is, substitutes the intended mother to bear the pregnancy for her.
Suggested for you: What Is a Surrogate Mother? – Everything You Should Know.