To deem is a verb that means to view as or judge. Your parents or boss may deem something necessary that you don't, like coming home by midnight or working late.
Deem comes from the Old English word, deman — meaning "act as a judge." If you deem something, you essentially become the judge of whether it has a characteristic. For example, if you deem your little sister's jokes to be too ridiculous to listen to, you will probably just walk away.