This adjective points at growth over time — a child’s mind and body, a skill, an animal, a product, even a whole science. Teachers, doctors, and researchers use it a lot: developmental stages, developmental psychology, a developmental problem. It is a school-and-clinic word more than a kitchen-table word, though actually parents hear it at the paediatrician’s all the time. It does not mean “still being built” in the way developing does (a developing country is one that is not rich yet). Developmental asks a quieter question: about the process of growing.