(sciences) Used loosely, a tentative conjecture explaining an observation, phenomenon or scientific problem that can be tested by further observation, investigation and/or experimentation. As a scientific term of art, see the attached quotation. Compare to theory, and quotation given there.

(statistics) A hypothesis set up to be nullified or refuted in order to support an alternative hypothesis. When used, the null hypothesis is presumed true until statistical evidence in the form of a hypothesis test indicates otherwise. Therefore, the null and the alternative hypothesis must be mutually exclusive and exhaustive.

(statistics) A rival hypothesis to the null hypothesis. Their likelihoods are compared by a statistical hypothesis test.

(set theory) The hypothesis which states that any infinite subset of ℝ must have the cardinality of either the set of natural numbers or of ℝ itself.

(historical, chemistry) An early 19th-century attempt to explain the existence of the various chemical elements: the hypothesis that, since the atomic weights for known elements appeared to be whole multiples of the atomic weight of hydrogen, the hydrogen atom was the only truly fundamental object, and the atoms of other elements were actually groupings of various numbers of hydrogen atoms.

The hypothesis that the Torah (the first five books of the Bible) was derived from originally independent, parallel, and complete narratives, which were subsequently combined into the current form by a series of redactors.

The hypothesis that God exists, seen as something able to be analyzed by a scientific approach rather than accepted by faith.

(linguistics) The hypothesis that, for adult second language learners, acquiring grammatical properties and interfacing between them (for example, between syntax and semantics) should not be problematic, but that interfacing between these and an external component, such as pragmatics or discourse information, will prove to be very difficult.

(creationism) The proposition that God created the universe within the past few thousands of years but also introduced false evidence that the universe is of great age.

The hypothesis that reality is in fact a simulation (such as a computer program) of which we, the simulants, are unaware.