Decorum is proper and polite behavior. If you let out a big belch at a fancy dinner party, you're not showing much decorum.
This noun is from Latin decōrus "proper, becoming, handsome," from décor "beauty, grace," which is also the source of English décor. The corresponding adjective is decorous, meaning "well-behaved in a particular situation." Both decorum and decorous are often used to describe behavior in a classroom or courtroom.