This is the adverb twin of developmental. You put it next to a verb or an adjective to say “when we look at how someone or something grows.” The phrases students meet first are developmentally delayed and developmentally appropriate (right for a child’s age). It is a formal, medical, educational word. You would not use it about homework being late, or about a city “developing.” Though actually, once you hear it at school or in a clinic, it tends to stay inside those set phrases.