Imminence means that something (typically something negative or dangerous) is about to happen. It is an impending event and it is implied that it is going to happen soon. Most frequently imminence refers to something dangerous that is about to happen.
For example, fast moving dark clouds, thunder, and lightning indicate the imminence of a big storm that is about to occur. In legal venues imminence refers to the likelihood that a dangerous or harmful event was going to happen to a person. In regards to self-defense the imminence requirement refers to the state and situation at which an individual was about to be harmed. The imminence requirement helps ensure the validity of a self-defense claim: a self-defense claim is not justified if no imminent threat to the individual was occurring.