Synonyms for Speculation (same or very similar meaning)
WordNet sense 1 (an investment that is very risky but could yield great profits):
venture
WordNet sense 2 (a message expressing an opinion based on incomplete evidence):
conjecture, guess, hypothesis, supposition, surmise, surmisal
WordNet sense 3 (continuous and profound contemplation or musing on a subject or series of subjects of a deep or abstruse nature):
meditation
From the ODE community, based on WordNet
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