Your pelvis is the set of large, wide, curved bones at the base of your spine, the place your legs join on to. It is not one bone: several bones grow together as you get older and make a kind of bowl, which holds some of your insides and takes the weight of your upper body when you stand. The old Latin word meant 'basin', and that picture still helps. People often say hip when they mean this whole area; though actually the hip is more the joint at the side, and everyday English is messy about that. In class and in hospitals, pelvis is the careful word. You will also hear pelvic (the adjective) much more than pelvis itself in phrases.