A type of employee stock option with a tax benefit, when you exercise, of not having to pay ordinary income tax. Instead, the options are taxed at a capital gains rate. Although ISOs have more favorable tax treatment than non-qualified stock options (NSOs), they also require the holder to take on more risk by having to hold onto the stock for a longer period of time in order to receive the better tax treatment. Also, numerous requirements must be met in order to qualify as an ISO.