osteoblastoma /-blastō″mə/ pl. osteoblastomas, osteoblastomata, a small, benign, fairly vascular tumor of poorly formed bone and fibrous tissue, occurring most frequently in the vertebrae, femur, tibia, or bones of the upper extremities in children and young adults. The tumor may cause pain, erosion, and resorption of native bone. Excision is the preferred treatment. Also called osteoid osteoma.