A hypothesis is either a suggested explanation for an observable phenomenon, or a reasoned prediction of a possible causal correlation among multiple phenomena. In science, a theory is a tested, well-substantiated, unifying explanation for a set of verified, proven factors. A theory is always backed by evidence; a hypothesis is only a suggested possible outcome, and is testable and falsifiable.
|Definition||A suggested explanation for an observable phenomenon or prediction of a possible causal correlation among multiple phenomena.||In science, a theory is a well-substantiated, unifying explanation for a set of verified, proven hypotheses.|
|Based on||Suggestion, possibility, projection or prediction, but the result is uncertain.||Certainty, evidence, verification, repeated testing.|
|Is well substantiated||No||Yes|
|Data||Usually based on very limited data||Based on a very wide set of data tested under various circumstances.|
|Instance||Specific: Hypothesis is usually based on a very specific observation and is limited to that instance.||General: A theory is the establishment of a general principle through multiple tests and experiments, and this principle may apply to various specific instances.|
edit Examples of Theory and Hypothesis
Theory: Einstein's theory of relativity is a theory because it has been tested and verified innumerable times, with results consistently verifying Einstein's conclusion. However, simply because Einstein's conclusion has become a theory does not mean testing of this theory has stopped; all science is ongoing. See also the Big Bang theory, germ theory, and climate change.
Hypothesis: One might think that a prisoner who learns a work skill while in prison will be less likely to commit a crime when released. This is a hypothesis, an "educated guess." The scientific method can be used to test this hypothesis, to either prove it is false or prove that it warrants further study. (Note: Simply because a hypothesis is not found to be false does not mean it is true all or even most of the time. If it is consistently true after considerable time and research, it may be on its way to becoming a theory.)
This video further explains the difference between a theory and a hypothesis:
edit Common Misconception
People often tend to say "theory" when what they're actually talking about is a hypothesis. For instance, "Migraines are caused by drinking coffee after 2 p.m. — well, it's just a theory, not a rule."
This is actually a logically reasoned proposal based on an observation — say 2 instances of drinking coffee after 2 p.m. caused a migraine — but even if this were true, the migraine could have actually been caused by some other factors.
Because this observation is merely a reasoned possibility, it is testable and can be falsified — which makes it a hypothesis, not a theory.