Referral bias is a type of selection bias.
People who are referred into studies are frequently different from those who are not, meaning that the results of a trial may not generalize well to the general population. (Referral bias should also be considered for studies done in populations at referral centers, which are often very different from the population of patients in the community.)
This post is part of a series of posts on bias in medical research. You can find the whole bias catalogue here.