spec‧u‧la‧tion [uncountable and countable]
when you guess about the possible causes or effects of something without knowing all the facts, or the guesses that you make
There is speculation that the president is ill.
speculation about the future
the witness's statement was pure speculation (=not based on any facts)
wild/idle speculation (=speculation that is unlikely to be true)
when you try to make a large profit by buying goods, property, shares etc and then selling them: