Scientific theory

A scientific theory is "a proposed description, explanation, or model of the manner of interaction of a set of natural phenomena, capable of predicting future occurrences or observations of the same kind, and capable of being tested through experiment or otherwise falsified through empirical observation. It follows from this that for scientists "theory" and "fact" do not necessarily stand in opposition. For example, it is a fact that an apple dropped on earth has been observed to fall towards the center of the planet, and the theory which explains why the apple behaves so is the current theory of gravitation." [W]

Also, "A coherent statement or set of statements that attempts to explain observed phenomena, which has testable implications, and which is well tested and widely accepted as true." (wiktionary)

This should not be confused with the common usage of the word "theory", referred to here as informal theory.