hypothesis (plural hypotheses; from the ancient Greek ὑπόθεσις hypothesis, composed of hypo, “under” and thesis, “position”, or supposition) is the premise underlying reasoning or a demonstration; in other words is a suggested explanation for an event, which one can test.

Originally, the meaning of the word indicated a mathematical method capable of simplifying the calculations, or a plausible but not necessarily true idea. With this meaning, Cardinal Bellarmine used the word when he warned Galileo Galilei not to treat the movement of the Earth as real, but to assume the Copernican system only as a hypothesis.

In common usage, a hypothesis is a tentative idea whose value must be ascertained. The hypothesis, therefore, requires an effort by researchers to confirm or deny it. In the hypothetical-deductive method, a hypothesis should be falsifiable, that is, it should be possible to declare it false, usually through observation and consequent formulation of another logical hypothesis.

In a scientific and epistemological sense, a hypothesis is the first formulation of a law, not yet tested or experimental in itself, intended to provide – along with a description of particular events and rules of deduction – an explanation or forecast of certain phenomena; this provisional formulation serves to determine further research from which the hypothesis itself may or may not have confirmation; if the hypothesis refers to an organic complex of laws and if it happens that, after experimental confirmation and technical reworking, the hypothesis assumes a complete and comprehensive form, it will take the name of the theory.