From the Latin omni - 'all' and potent - 'power', literally "all powerful".

In the Philosophy of Religion, those defending Theism define ‘god’ as a being with three fundamental characteristics: perfect knowledge, perfect power, and perfect goodness.

The perfect power of God does not mean that God can do anything without limit; this leads to sophomoric contradictions. For example, God cannot make a five-sided square. This is not an imperfection in God’s power since four-sidedness is essential to squareness; to increase or decrease the number of sides would destroy the thing (and make it something else). Hence, a more accurate definition of omnipotence would be, God knows do everything that is doable.

A similar distinction holds for the divine attribute of power - omniscience. It does not mean God knows everything, it means anything that can be known is within the knowledge of God.