An exchange-traded (though customizable) option that allows the parties to specify the strike price and expiration date according to their own requirements. This sets it apart from a standard exchange traded option in which trade takes place only for a predefined number of strikes and specific dates. Flexible exchange traded options (also called FLEX options) were first introduced by the CBOE in 1993 in an attempt to target the over-the-counter market of equity options.

This option differs from a standard equity option in many aspects, especially that it (an FLEX option) allows traders to customize its exercise price (strike increments for equity options are 1/8, while for stock index FLEX options, exercise prices can take any index value). Expirations dates for a FLEX option range between one business day and 5 years from the trade date (provided that such dates don’t fall on the third Friday of the month or two business days preceding or following that date.