the smallest possible amount of something; a tiny bit or a very small quantity.
From Greek 'iota,' the ninth and smallest letter of the Greek alphabet. The word came to mean a small amount because the letter itself looked small when written.
The Greek letter iota (ι) is literally the smallest letter in the Greek alphabet, so when English speakers needed a word for 'a tiny bit,' they borrowed iota—language often uses what's physically smallest to mean 'smallest amount'!
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