An idea or opinion based on insufficiently conclusive evidence; a conjecture. (via dictionary.com) It is the noun form of the verb "surmise." For some reason not recognised as a word by "legitimate" dictionaries. Dictionary.com strangely thinks that the noun form of "surmise" is, again, "surmise", which is just silly. Potentially first used and thus invented by me.
If one makes a surmission, said surmission is generally incorrect, as per the definition of the word.