What is a "second-strike capability" in terms of the military?

A 'second-strike capability' is a potent phrase that packs a lot of punch when it comes to the military. In short, it signifies having the ability to launch a nuclear attack after being hit first - and hard - by an enemy country with a nuclear strike.

So it’s a direct “an eye for both eyes” retaliation for an uninstigated nuclear attack. This might sound like common sense, but if a nation had only one arsenal of weapons or missiles, they would have no choice but to retaliate in kind before their enemies fire off their next barrage. Should this occur, any chance of ending a conflict without full-blown war is out the window. So having a second-strike capability is seen as a preventative measure against anyone who might want to start a fight: