Omniscience is a property traditionally ascribed to God in monotheistic belief. This means that God knows all true statements and does not believe any false statements. This knowledge is de dicto (knowing that) and does not include procedural knowledge, knowledge de re (direct/causal knowledge) or experiential knowledge.
The traditional definition for omniscience is that God knows all true statements. This definition is disputed by those who claim that there are some true statements that no one, including God, can know. In that case, omniscience would be redefined as the knowledge of all truths that it is possible to know.
Omniscience and Human Freedom
There is some debate over the compatibility of divine foreknowledge and human freedom. Some believe that human freedom does not include the ability to choose between alternatives. They find no incompatibility between the traditionally defined omniscience and human freedom. Others believe that human freedom does include the ability to choose between alternatives. If this strong form of human freedom is not compatible with the traditional definition of omniscience, then one's position is either open theism or process theism. In either case, one could accept the redefined version of omniscience.
There is another option available to those who do not believe that God can know the future free choices of human beings. One can deny that there are true statements about the future free choices of human beings. Statements about future free choices could be all false, or they could be neither true nor false. In either case, one can accept the traditional definition of omniscience while denying that God knows the future free choices of human beings.
If one affirms that God knows the future free choices of human beings and that human freedom includes the ability to choose between alternatives, then there are two ways that one could attempt to resolve the difficulty. One of these ways is to deny that God's knowledge of an event is temporally prior to the event. If God is eternal (timeless) then there is no event that occurs after his knowledge. This solution is known as Boethianism. Another solution is to claim that God knows what every possible human being would choose in every possible circumstance. Using this knowledge and knowing the circumstances that human beings are in enables God to know the future. This position is known as Molinism.
Boethius. The Consolation of Philosophy, Book V.
Flint, Thomas.1998. Divine Providence: The Molinist Account. Ithaca: Cornell University Press.
Hasker, William. 1989. God, Time, and Knowledge. Ithaca: Cornell University Press.