An agnostic is a person who believes that the question of the existence or non-existence of God(s) is a fundamentally unanswerable question. They therefore neither believe in God, nor disbelieve.
A person who is undecided about whether God exists may nevertheless not be agnostic in the strict sense if they also believe that the existence of God is an answerable question. However, it should be noted that many people equate the term "agnostic" with such undecidedness.
An agnostic is not the same as an atheist.