Can anyone explain to me why Paget's disease means two completely different diseases? My dictionary says that it's a bone disease and a form of breast cancer.

The same man described two entirely different diseases. Here is a URL with a lot of information:

http://www.genomelink.org/paget/

The same man described two entirely different diseases

Three?

http://www.med.umich.edu/obgyn/vulva/pagets.htm