For those who are still not sure, let me give a quick run down on what core is. Core comprises of the abs, back and pelvis. Having a six-pack does not necessarily mean you have a strong core, because I have seen dozens of perfectly looking athletes with six-packs and poor core.
Understand that your body does not rely on one single muscle to generate balance, force, power or speed. Instead, it works collectively in drawing energy from all the connecting muscles necessary to produce the desired movement or force.