An iota is an infinitesimal amount of something. The word iota is used synonymously with the etymologically related jot, and by its oldest definition refers to the 9th letter of the Greek alphabet.

// There's not an iota of doubt regarding the defendant's guilt.

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"Six years and one pandemic later, the owners’ standards have not slipped one iota." — Kitty Greenwald, Vogue, 22 Nov. 2022

Feel free to jot this down so you don’t forget: the words iota and jot share a lot more than just a common meaning—both ultimately come from the same word. When Latin scholars transcribed the Greek name of the ninth letter of the Greek alphabet, they spelled it as either iota or jota (the letters i and j were simply variants of each other), and these spellings eventually passed into English as iota and jot. Since the Greek letter iota is the smallest letter of its alphabet, both words eventually came to be used in reference to very small things.