From Envirowiki

In science, a hypothesis is an educated guess at why and how a process happens or a thing exists. The scientific method takes a number of hypothesis, and tests them experimentally to rule out competing hypotheses, until one hypothesis appears as the most viable.

Once a hypothesis is accepted as the most probable truth, it becomes a theory. A theory is generally treated as the best understanding of reality, given the available information, but is in no way considered immutable. A theory is usually continually tested, until some experimental results prove it wrong, at which point a new hypothesis must be formulated.

Notable well-tested theories include Newton's theory of gravity, Einstein's theory of special relativity, and Darwin's theory of evolution.