A word that means "all-knowing," this is usually applied to God.
There is some dispute about what "all" actually means. Does God literally know everything?
It is also difficult for apologists to demonstrate an infinite God from finite phenomena.
Does god know π (pi) and all other irrational numbers for example to an infinite number of decimal places? If so God must be infinitely complex and statements by theologians that God is simple are untrue. Incidentally God or whoever wrote 1 Kings 7:23 did not know π, there π is given as exactly 3.  Is π 3.14159etc or is π exactly 3? Teaching the controversy would confuse some school children while the more intelligent and older children would come to doubt the Bible.
Free will and predestination
Does god know not only everything that has happened, but that also might happen? If so, then in what sense does free will exist? If all your decisions are known by God before you make them then it's not clear if you are really making any "choices" since all the things that you do are predestined.
In a universe with a truly omniscient God, the Calvinist doctrine of predestination would have to be true. This carries its own set of issues, as it implies that God intentionally creates people already knowing that they will wind up in hell, see Maltheism.
Fundamentalist Christians believe in the infallibility of the Bible, yet there are arguable contradictions to the issue of God's omniscience.
- "The attributes assigned to "God" by post-biblical theologians -- such as omniscience and immutability -- are simply not attributes possessed by the character Yahweh as drawn in biblical narratives. Indeed, on several occasions Yahweh is explicitly described as changing his mind, because when it comes to human beings his learning curve is steep. Humans have free will; they act in ways that surprise him and he must change tack and respond. "
- And Jesus increased in wisdom and stature, and in favour with God and man.
- And Adam and his wife hid themselves from the presence of the Lord, amongst the trees of the garden.
- And they said unto him, Where is Sarah thy wife? And he said, Behold, in the tent.
- And the Lord said unto Satan, Whence comest thou? Then Satan answered the Lord, and said, from going to and fro in the earth, and from walking up and down in it.
- God left him, to try him, that he might know all that was in his heart.
Many more are cited in the Skeptic's Annotated Bible.