Growth factor: A substance that promotes the growth of cells. Growth factors include epidermal growth factor (EGF), fibroblast growth factor (FGF), erythropoietin (EPO), hematopoietic cell growth factor (HCGF), platelet-derived growth factor (PDGF), stem cell factors, and neurotrophins. Growth factor is produced by normal cells during embryonic development, tisue growth, and wound healing. Tumors, however, produce large amounts of growth factors.