From the Latin omni - 'all' and sciens - 'knowing', literally "all knowing".

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 knowledge of God does not mean that God knows everything without limit; this leads to sophomoric contradictions. For example, God does not know what the largest number is. This is not an imperfection in God’s knowledge since there is no such entity to be know. Hence, a more accurate definition of omniscience would be, God knows every true proposition (and therefore by default, every false one as well), or God knows everything that is knowable.

A similar distinction holds for the divine attribute of power - omnipotence. It does not mean God can do anything, it means anything that can be done is within the power of God.