Help please? i want to know what are illdefined osteolytic lesions in bone metastases?

Bone mets: Bone metastases can cause focal areas of bone loss or destruction, called "osteolytic". If they cause a focal bone response of increased density or sclerosis, they are called "osteoblastic". "Ill-defined" means that the borders of the bone met are not clearly visualized, which may imply a faster growing or more aggressive met. Slow growing or indolent lesions tend to develop well defined margin