supposition

1. The act of supposing.
2. Something supposed; an assumption.

noun

1. A judgment, estimate, or opinion arrived at by guessing: conjecture, guess, guesswork, speculation, surmise. See opinion.
2. Something taken to be true without proof: assumption, postulate, postulation, premise, presupposition, theory, thesis. See reason/unreason.

Definition: guess, belief
Antonyms: fact, knowledge, proof, reality

(1) The concept, proposed by Rameau, that chords of the 9th and 11th, among others, arise from a 7th chord by placing a ‘supposed’ bass one or two 3rds below the Fundamental bass (e.g. in the chord f-a-c′-e′-g′-b′ the fundamental bass is c′ and the ‘supposed’ bass is f).

(2) The imagining of a different clef sign, or the existing one differently placed, in a piece of music, thus enabling a player to transpose music that he is playing.

The noun has 3 meanings:

Meaning #1: a message expressing an opinion based on incomplete evidence
  Synonyms: guess, conjecture, surmise, surmisal, speculation, hypothesis

Meaning #2: a hypothesis that is taken for granted
  Synonyms: assumption, supposal

Meaning #3: the cognitive process of supposing
  Synonym: supposal