What is All Or None?

Legal Definition
All or none (AON) is a finance term used in investment banking or securities transactions that refers to "an order to buy or sell a stock that must be executed in its entirety, or not executed at all". Partial execution is not acceptable; the order will execute "only if there are enough shares available in a single transaction to cover it".

An all-or-none clause in an underwriting contract or investment prospectus gives a securities issuer the right to cancel an issue in its entirety if the underwriting is not fully subscribed.

AON orders are similar to fill or kill (FOK) orders, but the former focuses on "complete vs. partial fulfillment", whereas the latter hinges on the immediacy of the transaction.
-- Wikipedia
Legal Definition
An order where a broker buys or sells the parts of an order that can be filled. If this does not occur the sale is pending. Canceled orders cannot be filled. AKA fill or kill.