Launch on warning is a policy of (typically nuclear) weapon use. Under such a
policy, a nation explicitly reserves to itself the right (or announces its intent) to launch its own weapons as soon as it has relatively sound evidence that an opponent has launched an attack on it. However, the main purpose of such a policy is to ensure that one's own weapons are launched before any incoming
warheads can
detonate and possibly destroy them before use; ergo, there is significant
time pressure on
decision makers to evaluate any warning that comes in, with the ultimate penalty if they guess wrong. The U.S. has never officially declared a launch on warning policy, as it has declared its willingness to use
nuclear weapons first.
Contrasting policies would be 'No First Use' or 'Launch Under Attack.'