The rate at which options granted under a stock option plan become exercisable by the option holder. Most stock option plans provide that options vest (and therefore become exercisable by the option holder) over a period of years so that the company gets the benefit of extended employment and performance from the option holder. A common pattern is for options to vest in equal percentages over three to five years, usually on the anniversary date of the option grant. If the option holder's relationship with the company ceases, then the option holder forfeits the options that have not yet become vested.