A "second strike" refers to a military strategy where a country retains the capability to respond to a nuclear attack with its own nuclear weapons. This ensures that even if an adversary launches a first strike, the targeted nation can still retaliate effectively, deterring the initial attack. This concept is crucial in the context of nuclear deterrence, as it aims to prevent conflicts by assuring potential aggressors that any attack would lead to devastating consequences. Countries like the United States and Russia maintain second strike capabilities to enhance their national security and stability in international relations.