An all-aspect missile is one which is able to track a target no matter which way the target faces relative to the missile. In other words, an all-aspect missile can be launched against a target in a tail-chase engagement, in a head-on engagement, in a side-on engagement, from above, from below, etc.
This is as opposed to older infra-red homing missiles which were only able to track the hot engine exhaust of an aircraft if the aircraft's engine exhaust was pointing towards the missile seeker, and thus were only successfully used in tail-chase engagements.
