Question

Which word is closest in meaning to the word "conjecture"?

Answer 1

It's a word to use when you are not sure of something and have to "guess or surmise."  

hope it helps

Answer 2

Conjecture is a noun that is defined as "the formation or expression of an opinion or theory without sufficient evidence for proof.  An opinion or theory so formed or expressed; guess; speculation."

Some synonyms of conjecture include: guess, presumption, suposition, conclusion, hypothesis, inference.

As a verb, conjecture means to conclude or suppose from grounds or evidence insufficient to ensure reliability.

So, the closest word in meaning would be guess.