A denunciation is a public reprimand in which you are told that you've done wrong. If you get caught skipping school, expect denunciation-by teachers, your principal, parents and maybe even your friends.
When you harshly criticize someone or something in front of other people, it's a denunciation, like an opinion piece that appears in a newspaper. Sometimes denunciation has the meaning of telling on or exposing someone's misdeeds, like a juicy, book-length denunciation of a famous guitarist written by a former bandmate. The Latin root word is denuntiare, "to announce, proclaim, denounce, or command."