Decalcification was the loss of calcium salts from a bone or tooth.
In 2267, Dr. Leonard McCoy performed an intensive scan on the natives of Gamma Trianguli VI and found no indications of aging conditions such as decalcification and arteriosclerosis. (TOS: "The Apple")
In 2373, Professor Gegen observed that the degree of bone decalcification in the remains of Hogan indicated that he had been dead for more than a year. (VOY: "Distant Origin")
In 2374, Chakotay began to suffer from bone decalcification within a matter of hours as a result of illicit Srivani experiments. (VOY: "Scientific Method")