发育 (fāyù) refers specifically to physical/biological development and maturation of living organisms (e.g., a child's body, a plant). It contrasts with 发展 (fā zhǎn), which is broader (social, economic, etc.), and 成长 (chéng zhǎng), which includes emotional/personal growth; 成熟 (chéng shú) describes the result (mature/ripe) rather than the process.