Some states have specifically mentioned self-defense can be used when the danger is a specific felony, such as rape, sodomy, kidnapping, and robbery. Reasonable force means that if someone slaps your face and you shoot them, that's unreasonable because you escalated force a bit too far when just returning a slap in the face might have been more reasonable. Imminent is a word meaning in progress or about to happen right now. You can't use self-defense for continuing and ongoing danger, although a few states have allowed this. Most states use an objective test to determine the sense of being imminent, and their statutes either spell out the grounds for a reasonable belief or they use a reasonable man standard.
Most states also adhere to the retreat doctrine .
This is the rule that defenders must have taken all means to avoid or escape before attacking in self-defense, although in some jurisdictions, the law views this as cowardly, and a standard called the true man doctrine is applied. Most states, however, adhere to the castle doctrine , the idea that when attacked in one's home, you should hold your ground and not be required to retreat. This is called defense of property . It's the basis for a lot of military justice, diplomatic immunity, and covers a number of things that police do, such as shooting looters after a disaster. The idea here is that there's a higher loyalty, and it's located in upholding the law.