hypothesis

American Heritage® Dictionary of the English Language, Fourth Edition

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

Century Dictionary and Cyclopedia

1. A condition; that from which something follows: as, freedom is the hypothesis of democracy.
2. A proposition assumed and taken for granted, to be used as a premise in proving something else; a postulate.
3. 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.
4. The conclusion of an argument from consequent and antecedent; a proposition held to be probably true because itsconsequences, 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. The word has always been applied in this sense to theories of the planetary system. Kepler held the hypothesis that Mars moves in an elliptical orbit with the sun in one focus, describing equal areas in equal times, the ellipse having a certain size, shape, and situation, and the perihelion being reached at a certain epoch. Of the three coördinates of the planet's position, two, determining its apparent position, were directly observed, but the third, its varying distance from the earth, was the subject of hypothesis. The hypothesis of Kepler was adopted because it made the apparent places just what they were observed to be. A hypothesis is of the general nature of an inductive conclusion, but it differs from an induction proper in that it involves no generalization, and in that it alfords an explanation of observed facts according to known general principles. The distinction between induction and hypothesis is illustrated by the process of deciphering a despatch written in a secret alphabet. A statistical investigation will show that in English writing, in general, the letter e occurs far more frequently than any other; this general proposition is an induction from the particular cases examined. If now the despatch to be deciphered is found to contain 26 characters or less, one of which occurs much more frequently than any of the others, the probable explanation is that each character stands for a letter, and the most frequent one for e: this is hypothesis. At the outset, this is a hypothesis not only in the present sense, but also in that of being a provisional theory insufficiently supported. As the process of deciphering proceeds, however, the inferences become more and more probable, until practical certainty is attained. Still the nature of the evidence re mains the same; the conclusion is held true for the sake of the explanation it alfords of observed facts. Generally speaking, the conclusions of hypothetic inference cannot be arrived at inductively, because their truth is not susceptible of direct observation in single cases; nor can the conclusions of inductions, on account of their generality, be reached by hypothetic inference. For instance, any historical fact, as that Napoleon Bonaparte once lived, is a hypothesis; for we believe the proposition because its effects—current tradition, the histories, the monuments, etc.—are observed. No mere generalization of observed facts could ever teach us that Napoleon lived. Again, we inductively infer that every particle of matter gravitates toward every other. Hypothesis might lead to this result for any given pair of particles, but never could show that the law is universal. The chief precautions to be used in adopting hypotheses are two: first, we should take pains not to confine our verifications to certain orders of effects to which the supposed fact would give rise, but to examine effects of every kind; secondly, before a hypothesis can be regarded as anything more than a suggestion, it must have produced successful predictions. For example, hypotheses concerning the luminiferous ether have had the defect that they would necessitate certain longitudinal oscillations to which nothing in the phenomenacorresponds; and consequently these theories ought not to be held as probably true, but only as analogues of the truth. As long as the kinetical theory of gases merely explained the laws of Boyle and Charles, which it was constructed to explain, it had little importance; but when it was shown that diffusion, viscosity, and condnctibility in gases were connected and subject to those laws which theory had predicted, the probability of the hypothesis became very great.
5. An ill-supported theory; a proposition not believed, but whose consequences it is thought desirable to compare with facts.

Wiktionary

1. n. 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.
2. n. general An assumption taken to be true for the purpose of argument or investigation.
3. n. grammar The antecedent of a conditional statement.

GNU Webster's 1913

1. n. 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.
2. n. (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.

WordNet 3.0

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