Periosteum: The periosteum is a dense membrane composed of fibrous connective tissue that closely wraps (invests) all bone, except that of the articulating surfaces in joints which are covered by a synovial membrane.

"Peri-" is a prefix borrowed from the Greek meaning "around or about" and "-osteum" also comes from the Greek, from "osteon" meaning "bone", so periosteum literally means "around the bone."