Speculation was a type of educated guess.
Starfleet's brig emergency evacuation protocol required convincing the computer's ethical protocols beyond speculation that release would be more likely to ensure a prisoner's survival than keeping the prisoner in captivity. (DIS: "Battle at the Binary Stars")
In 2265, while having a meeting in the briefing room with Lee Kelso, Elizabeth Dehner, and Spock, Captain James T. Kirk explained, " It is my duty, whether pleasant or unpleasant, to listen to the reports, observations, even speculations, on any subject that might affect the safety of this vessel, and it's my science officer's duty to see I'm provided with that." (TOS: "Where No Man Has Gone Before")