something that it is essential to see, do, hear etc; (used to express an obligation or an order)

necessity, essential

the state, condition (e.g. a law of nature, a (serious) need, a social rule etc) that makes a certain result unavoidable; something that one cannot be, live etc without

need, essentiality

(used to tell a person about a present or past duty or obligation) (used to express an opinion, give advice etc about the present or past)

necessity, essential