Jot or tittle
A tiny amount.
The phrase 'jot or tittle' is somewhat tautological, as both jot and tittle refer to tiny quantities. It has passed into English via William Tindale's translation of the New Testament in 1526. It appears there in Matthew 5:18:
One iott or one tytle of the lawe shall not scape.
The more familiar language of the King James Version, 1611, renders that verse as:
For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled.
A jot is the name of the least letter of an alphabet or the smallest part of a piece of writing. It is the Anglicized version of the Greek iota - the smallest letter of the Greek alphabet, which corresponds to the Roman 'i'. This, in turn, was derived from the Hebrew word jod, or yodr, which is the the smallest letter of the square Hebrew alphabet. Apart from its specialist typographical meaning, we still use the word jot more generally to mean 'a tiny amount'. Hence, when we have a brief note to make, we 'jot it down'.
A tittle, rather appropriately for a word which sounds like a combination of tiny and little, is smaller still. It refers to a small stroke or point in writing or printing. In classical Latin this applied to any accent over a letter, but is now most commonly used as the name for the dot over the letter 'i'. It is also the name of the dots on dice. In medieval calligraphy the tittle was written as quite large relative to the stem of the 'i'. Since fixed typeface printing was introduced in the 15th century the tittle has been rendered smaller.
The use of the word 'dot' as a small written mark didn't begin until the 18th century. We may have been told at school to dot our i's; Chaucer and Shakespeare would have been told to tittle them.