Osteomalacia

Osteomalacia is a softening of the bones.  It is usually caused by a nutritional deficiency; a shortage of calcium, phosphorus or, most often vitamin D.  In children, it is known as rickets and is more serious, so the term is largely used to describe the milder condition that affects adults.  
In all cases, the condition can be traced to low vitamin D levels.  However, this can often be a secondary condition to other existing conditions rather than a lack of dietary vitamin D.  Most commonly, it can be caused by diseases of the kidneys or those that cause malabsorption.
Osteomalacia at Wikipedia