A hypothesis is an educated guess that a scientist makes in order to explain an observed phenomenon. A properly stated hypothesis is testable, meaning that someone could design an experiment or seek additional evidence to disprove its claim. A hypothesis that undergoes many tests and yet remains a valid explanation becomes a theory.
Ideally, a stated hypothesis should be accompanied by the "null hypothesis", a prediction of what would occur if the hypothesis is not true. Tests should be conducted of both the hypothesis and its null hypothesis.