Density is a measure of how compact and solid an object (like bone) is compared to its size. An object with high density is heavy and usually strong for its size; an object with low density is lighter and usually unable to bear much weight.
For example, osteoporosis causes the internal structure of bones to become less compact, or to have lower density. This makes bones more porous and weaker than normal, healthy bones.
WebMD Medical Reference from Healthwise