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

(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.

(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 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.

(biology) A hypothesis attempting to explain why most hosts of brood parasites care for the parasite's nestlings. Since the host's nest is attacked if the parasite's egg is detected and rejected, this indirectly enhances selective pressures favouring aggressive parasite behaviour that may result in positive feedback between mafia-like parasites and compliant hosts.

Synonym of conspiracy theory

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.