Jot (iota)
ἰῶτα / י
Greek · iōta
Matthew 5:18 — "one jot or one tittle shall in no wise pass from the law."
"Jot" is the English spelling of iota, the Greek name for the letter i — and Jesus is using it to translate yod (י), the smallest letter of the Hebrew alphabet, a stroke about the size of an apostrophe.
His point is about preservation: not even the smallest letter of God's word will be lost. It is the one place in the Bible where Jesus himself makes an argument from the shapes of letters — and the argument is that every stroke is kept.
English still keeps the word: to "jot something down" is to write the smallest possible note.