“small island,” from M.E. eyt, from O.E. igeoð, dim. of eg, ig, ieg “island” (see island). Ending infl. by Fr. dim. suffix -ot.

An ait (or eyot) is a small island found in the middle of a river or lake. It is especially used to refer to islands found on the River Thames in England.

Aits are typically formed by the deposition of sediment in the water, which accumulates over a period of time. An ait is characteristically long and narrow, and may become a permanent island. However, aits may also be eroded: the resulting sediment is deposited further downstream and could result in another ait. A channel with numerous aits is called a braided channel.

The words “ait” and “eyot” are not common in modern English, although a few famous writers have used it, including J. R. R. Tolkien in his Lord of the Rings books, and Charles Dickens in Bleak House. More recently, it was used by Terry Pratchett in the first of the Discworld books, The Colour of Magic.

 Source: Wikipedia.org.