The terms are interchangeable. A breast lump is stationary and your doctor will send you for aspiration of the lump to see if it's solid or full of fluid. Fluid is good and means it's non cancerous in most cases. A mass means that there is one or more lumps in a 'cluster' or a large spot in the breast that is spreading through the breasts into lymph nodes and possibly into the chest bones. We are not doctors on this site, so if anyone should be worried whether they have cancer of their breast(s) they should always talk to their physician. Also get the physician to teach you the correct way to give yourself a self examination of your breasts (which should be done at least once a week and generally after a shower.) REMEMBER don't do a self examination just before or while on your periods because you could feel a lump or even ridges which are generally nothing to worry about and are strictly hormonal. These will disappear when you finish your periods.