Omniscience is the ability to know everything there is to know.
Some have questioned whether omniscience is compatible with free will; specifically, Calvinists have argued that God knows whether a person will be saved before he is born. If God has foreknowledge, then only one path (known, albeit, only to God) can be chosen and therefore allows only one action can be taken; if God had seen that the man were bound to heaven, for example, he could not be but virtuous, while were he bound to hell he must, by definition, sin.
However, some believe knowledge of what the outcome of a man or woman's free will will be does not preclude his or her freedom. They contend that it is possible for God to know how a man or woman will exercise their free will and at the same time not interfere with it.
Some believe that this creates a logical paradox, but the paradox hinges on the concept of time. That is, if God knows what a person will choose before the person makes that choice, how does the person actually have a free choice about that when the time comes? But many Christians believe that God exists outside of time, thus time does not pass for God. He is not only everywhere at once, but in all of time at once. Consequently, the time factor of the apparent paradox is removed, and the paradox disappears. That is, God does not know what the person will choose before they do so. Rather, God has always known what the person did freely choose in that person's future. Put simply, if there were to be two Omnisciences, He would be both.
This, however, raises the question in many people's minds as to why God would allow evil things, knowing that they would arise. The problem with this question is that it assumes that God would not have had some purpose in allowing evil to arise.