It stands for all-or-none exercise; a style of exercise for options whereby the holder cannot exercise only on part of a basket or portfolio of underlying assets. Instead, exercise has to take place on the entire collection of assets at the same time. Otherwise, exercise would be impossible.
For example, in a basket currency option, a collection (basket) of currencies is treated together as a portfolio. A basket currency option gives the buyer the right, without the obligation, to receive certain currencies in exchange for a base currency, either at the spot market rate or at a predetermined rate of exchange, all subject to an all-on-none exercise (AON exercise).
