supposition

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

• noun The act of supposing.
• noun Something supposed; an assumption.

from The Century Dictionary.

• noun The act and mental result of hypothetical inference; that act of mind by which a likelihood is admitted in a proposition on account of the truth of its consequences; a presumption.
• noun The act and mental result of formulating a proposition, without reference to its truth or falsity, for the sake of tracing out its consequences; a hypothesis.
• noun In logic, the way in which a name is to be understood in a given proposition, in reference to its standing for an object of this or that class.
• noun Substitution.
• noun See the adjectives.

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

• noun The act of supposing, laying down, imagining, or considering as true or existing, what is known not to be true, or what is not proved.
• noun That which is supposed; hypothesis; conjecture; surmise; opinion or belief without sufficient evidence.

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

• noun Something that is supposed; an assumption made to account for known facts, conjecture
• noun The act or an instance of supposing

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

• noun a message expressing an opinion based on incomplete evidence
• noun a hypothesis that is taken for granted
• noun the cognitive process of supposing