The most used definition of "denunciation" is the act of denouncing someone, or condemning them in public. For example, a person would be performing denunciation by condemning someone else in a newspaper or other public forum.
In the legal system, the act of "denunciation" means that someone is accused of a crime before a tribunal or some kind of public prosecutor or court.
The act of "denunciation" can also mean to revoke or terminate an existing treaty. Certain treaties insert the act of denunciation into the treaty if some event happens, such as a number of countries no longer participating in the treat.
An older definition of "denunciation" which is rarely used is to warn someone of an impending threat or danger. However, because of the regular use of the above definitions, this definition is now considered archaic and is not in general use.
The word "denunciation" comes from the word "denunciate" which is essentially a Latin version of the word "denounce". As such, many definitions of denunciation prominently feature the act of denouncing someone.