conjecture

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

• noun Opinion or judgment based on inconclusive or incomplete evidence; guesswork.
• noun An opinion or conclusion based on guesswork.
• intransitive verb To judge or conclude by conjecture; guess.
• intransitive verb To make a conjecture.

from The Century Dictionary.

• noun The act of forming an opinion without definite proof; a supposition made to account for an ascertained state of things, but as yet unverified; an opinion formed on insufficient presumptive evidence; a surmise; a guess.
• noun Suspicious surmise; derogatory supposition or presumption.
• noun Synonyms Supposition, hypothesis, theory.
• To form (an opinion or notion) upon probabilities or upon slight evidence; guess: generally governing a clause.
• Synonyms Imagine, Conjecture, Surmise, Guess, Presume, fancy, divine. Imagine literally expresses pure speculation, and figuratively expresses an idea founded upon the slightest evidence: as, I imagine that you will find yourself mistaken. Conjecture is something like a random throw of the mind; it turns from one possibility to another, and perhaps selects one, almost arbitrarily. Surmise has often the same sense as conjecture; it sometimes implies a suspicion, favorable or otherwise: as, I surmise that his motives were not good. Guess suggests a riddle, the solution of which is felt after by the mind—a question, as to which we offer an opinion, but not with confidence, because the material for a judgment is confessedly insufficient. To presume is to base a tentative or provisional opinion on such knowledge as one has, to be held until it is modified or overthrown by further information.
• To form conjectures; surmise; guess.

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

• noun An opinion, or judgment, formed on defective or presumptive evidence; probable inference; surmise; guess; suspicion.
• transitive verb To arrive at by conjecture; to infer on slight evidence; to surmise; to guess; to form, at random, opinions concerning.
• intransitive verb To make conjectures; to surmise; to guess; to infer; to form an opinion; to imagine.

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

• noun formal A statement or an idea which is unproven, but is thought to be true; a guess.
• noun formal A supposition based upon incomplete evidence; a hypothesis.
• noun mathematics, philology A statement likely to be true based on available evidence, but which has not been formally proven.
• noun obsolete Interpretation of signs and omens.
• verb formal, intransitive To guess; to venture an unproven idea.

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

• verb to believe especially on uncertain or tentative grounds
• noun reasoning that involves the formation of conclusions from incomplete evidence
• noun a hypothesis that has been formed by speculating or conjecturing (usually with little hard evidence)
• noun a message expressing an opinion based on incomplete evidence