The Collaborative International Dictionary of English v.0.48:

Osteoblast \Os"te*o*blast\, n. [Osteo- + -blast.] (Anat.)
   One of the protoplasmic cells which occur in the osteogenetic
   layer of the periosteum, and from or around which the matrix
   of the bone is developed; an osteoplast.
   [1913 Webster]

WordNet (r) 3.0 (2006):

osteoblast
    n 1: a cell from which bone develops [syn: osteoblast, bone-
         forming cell]