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, although this creates a logical paradox.
This, however, raises the question as to why God would allow evil things, knowing that they would arise.