A prediction (Latin præ-, "before," and dictum, "something said") or forecast is a statement about a future event or about future data. Predictions are often, but not always, based upon experience or knowledge of forecasters. There is no universal agreement about the exact difference between "prediction" and "estimation"; different authors and disciplines ascribe different connotations.
Future events are necessarily uncertain, so guaranteed accurate information about the future is impossible. Prediction can be useful to assist in making plans about possible developments.
I've never had a good understanding of this. Say I am dissolving 20g sodium chloride into 1L of water. Is the only way to theoretically predict the final volume of solution to consult density tables?
edit: I realize this isn't a practical situation, and that typically you are making a solution...