Why is the aeolian mode called the relative minor?

I've been trying to find the exact answer to this question. It seems everywhere I look the aeolian mode is just referred to as the relative minor, but it is never explained why other than the fact that it shares the same notes as the major scale. What I don't understand about that is that all the other modes within the same scale also use the same notes. I figure I'm missing something. Thanks in advance!