General synonyms:
* Speculation: This is a very common synonym, suggesting a guess based on incomplete information.
* Guess: A simple and direct synonym.
* Assumption: This implies a belief taken for granted without proof.
* Hypothesis: More formal, suggesting a proposed explanation that can be tested.
* Surmise: This implies a tentative conclusion based on evidence, but not necessarily conclusive.
* Inference: A conclusion reached on the basis of evidence and reasoning.
* Theory: A more formal and systematic explanation, often based on a set of hypotheses.
More specific synonyms:
* Supposition: A tentative belief, often based on little evidence.
* Presumption: A belief that is assumed to be true without proof, often with a negative connotation.
* Conjecture: A guess, often based on little evidence, and typically with a sense of uncertainty.
The best synonym will depend on the context of your writing.