hypothesis

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A tentative explanation for an observation, phenomenon, or scientific problem that can be tested by further investigation.
• noun Something taken to be true for the purpose of argument or investigation; an assumption.
• noun The antecedent of a conditional statement.

from The Century Dictionary.

• A condition; that from which something follows: as, freedom is the hypothesis of democracy.
• A proposition assumed and taken for granted, to be used as a premise in proving something else; a postulate.
• A supposition; a judgment concerning an imaginary state of things, or the imaginary state of things itself concerning whose consequences some statement is made or question is asked; the antecedent of a conditional proposition; the proposition disproved by reductio ad absurdum.
• The conclusion of an argument from consequent and antecedent; a proposition held to be probably true because its consequences, according to known general principles, are found to be true; the supposition that an object has a certain character, from which it would necessarily follow that it must possess other characters which it is observed to possess.
• An ill-supported theory; a proposition not believed, but whose consequences it is thought desirable to compare with facts.

from the GNU version of the Collaborative International Dictionary of English.

• noun A supposition; a proposition or principle which is supposed or taken for granted, in order to draw a conclusion or inference for proof of the point in question; something not proved, but assumed for the purpose of argument, or to account for a fact or an occurrence.
• noun (Natural Science) A tentative theory or supposition provisionally adopted to explain certain facts, and to guide in the investigation of others; hence, frequently called a working hypothesis.
• noun See under Nebular.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.
• noun general An assumption taken to be true for the purpose of argument or investigation.
• noun grammar The antecedent of a conditional statement.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a tentative insight into the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena
• noun a message expressing an opinion based on incomplete evidence
• noun a proposal intended to explain certain facts or observations