Deterrence is where physical defense meets up with the mental defense of Situational Awareness. (SA)
SA is the process of identifying threatening people and environments and then taking steps to avoid conflict. The goal with SA is to win the fight by never being in it.
Now you're not always going to be so lucky to avoid the fight, that's where good physical defense skills must be ready to be put into action, skills like combatives, concealed draws, and shooting fundamentals.
Where the two meet is called Deterrence. Deterrence is the communicating to the threat that you will not be a victim, and that you pose a lethal threat to the bad guy's safety. SA helped you to identify that threat and gives you the programmed language to try and get the threat to mentally disengage and having a good concealed draw or combatives skills gives you the ability to fend that threat off if it chooses to continue their crime. This drill will help you work that critical moment.