What is the definition or description of: weak bones?

Osteopenia. This may arise from a diet low in calcium. Calcium is required for normal bone metabolism. If insufficient then the resulting bones may be weaker than normal.
Weak bones. There are a few categories that make up the conditions commonly thought of as "weak bones." osteoporosis is a bone quantity issue; it is very common in older populations. Osteomalacia is a bone quality issue; it may be due to dietary issues (e.g. Lack of vitamins and/or minerals), chronic disease (e.g. Chronic kidney disease) or genetic disease (e.g. Familial rickets). See www.Aaos.Org for more.