The self defense

context: “It was an act of self-defense.” ✅ (most common) “She acted in self-defense.” ✅ (natural and widely used) “The self-defense claim was accepted by the court.” ✅ (when talking about a legal claim) “He used self-defense to protect himself.” (less natural; “He acted in self-defense” is better) If you mean “The self defense” as part of a sentence, please share the full sentence so I can help you phrase it correctly.