An abbreviation for all or none; an initial public offering (IPO) agreement that can be nullified by the lead underwriter if all of the shares are not subscribed. In other words, an AON agreement stipulates that, for the IPO to be completed, all shares must be sold. If not all shares are sold within a specified period of time, the offer is withdrawn and investors’ money will be refunded (while the company gets no financing at all). The underwriter assumes no responsibility for a cancelled offer.