Within the context of nuclear deterrence, what constitutes a 'second strike capability'?

A 'second strike capability' in nuclear deterrence refers to a nation's ability to absorb a first strike nuclear attack by an adversary and still retain sufficient nuclear forces to inflict unacceptable damage in retaliation. This capability is crucial for maintaining a stable deterrent relationship because it assures the attacking nation that launching a first strike would not eliminate the possibility of a devastating response, thereby discouraging the initial attack. A second strike capability typically involves a combination of survivable nuclear delivery systems, such as submarine-launched ballistic missiles (SLBMs), which are difficult to detect and destroy; mobile intercontinental ballistic missiles (ICBMs), which can be moved to avoid targeting; and hardened missile silos, designed to withstand a nuclear blast. The key is that these forces must be able to survive a surprise attack and still be capable of launching a counterattack. For example, a nation might maintain a fleet of nuclear submarines constantly patrolling the oceans, ensuring that at least some of them would survive a first strike and be able to launch their missiles in retaliation. The existence of a credible second strike capability is a cornerstone of Mutually Assured Destruction (MAD), where both sides possess the ability to destroy each other, regardless of who strikes first, thus creating a powerful incentive to avoid nuclear war altogether.