What’s the difference between certainty and certitude? My hunch was that they’re interchangeable, but it turns out that they have a slight but significant difference in connotation—of that I am certain.
Certainty and certitude, of course, share a root word: the Latin term certus, meaning “fixed” or “settled.” Certainty originally meant “pledge” or “surety,” then developed a sense of “something that is certain,” and certitude derives from a Latin word with the same meaning. But along the way, distinct connotations have emerged, though they are not always observed: A certainty can be something known based on fact, while sometimes, a certitude is something one is convinced of on the basis of faith. (The synonym conviction more obviously refers to what one believes rather than what one knows.) This is a useful distinction, but it’s unfortunately unlikely to ever emerge as one writers use consistently.
The antonyms differ in form: uncertainty, but incertitude. (Incertain and incertainty existed in Middle English but were supplanted by the un- forms.)
Other words based on certus include the adjectival and adverbial forms of certainty, certain and certainly, as well as the verb certify, meaning “confirm” or “vouch for”; the noun form is certification, and something that can be confirmed or vouched for is certifiable. (In colloquial usage, one who is, in jest or derision, thought eligible to be judged insane is referred to as certifiable.) Certificated is an adjective derived from the obsolete use of certificate as a verb. The noun certificate originally meant “action of certifying” but now has a dominant sense of “a document that provides certification.”
Ascertain (the prefix a, meaning “to,” followed by certain) once meant “assure” or “inform” but now means “determine.”
Concert and associated words are related to certain by way of the root word, though here, -cert has the sense of “strive.”