A surrogate, or substitute, refers to a person or thing that takes the place of another. In various contexts, it can mean a replacement for someone in a specific role or function. For example, a surrogate mother carries and gives birth to a child for another person or couple who cannot conceive. In a broader sense, surrogates can also apply to objects or concepts. For instance, a surrogate marker in medicine is a substitute measure used to infer the presence or progression of a disease. Surrogates help fulfill needs when the original is unavailable or unsuitable.