To deem something means to judge it or consider it to be a certain way. When your teacher deems your science project ready for the fair, she's decided it meets the requirements. When a referee deems a play out of bounds, he's making an official judgment call.

The word carries a sense of authority or finality. You might think your room is clean enough, but your parents may deem it still too messy for you to go outside and play. A judge might deem evidence inadmissible in court, meaning she's ruled it can't be used. A committee might deem a building unsafe, which means they've made an official determination about its condition.

Deem sounds formal because it often appears in official contexts: rules, decisions, and judgments. You're more likely to see it written than hear it in casual conversation. When something is deemed necessary, appropriate, or unacceptable, someone in a position to decide has made that call. The word suggests that the judgment matters and will affect what happens next.