Osteoclast: A cell that nibbles at and breaks down bone and is responsible for bone resorption.
Osteoclasts are large multinucleate cells (cells with more than one nucleus) that differentiate from another type of cell called a macrophage.
In normal bone, bone formation and bone resorption are closely coupled processes involved in the normal remodeling of bone. In osteoporosis, the net rate of bone resorption exceeds the rate of bone formation, resulting in a decrease in bone mass without a defect in bone mineralization. In women, osteoclast activity is increased because of decreased estrogen after the menopause. (Men with prematurely decreased testosterone may also have increased osteoclast activity.) These changes result in further net loss of bone. The amount of bone available for mechanical support of the skeleton eventually falls below the fracture threshold and one may suffer a fracture with little or no trauma.
The term "osteoclast" has a curious history. It was derived by putting osteo- (from the Greek osteon, bone) together with -clast (from the Greek klastos, broken). It was originally a surgical instrument used to fracture bones. The cell we call an osteoclast was then named an osotoclast to avoid confusing it with the surgical instrument. When the instrument became outmoded, the mantle of osteoclast fell upon the cell that resorbs bone.