Ossification: The process of creating bone, that is of transforming cartilage (or fibrous tissue) into bone.
The human skeleton initially consists largely of cartilage which is relatively soft and is gradually transformed into hard bone during infant and child development.
The verb corresponding to "ossification" is "ossify." Cartilage becomes ossified as it is converted into bone. Bone is osseous tissue. "Os" is a synonym for "bone." The Latin word "os" means "bone" as does the related Greek word "osteon."