Stock Option

A privilege, sold by one party to another, that gives the buyer the right, but not the obligation, to buy (call) or sell (put) a stock at an agreed-upon price within a certain period or on a specific date. In the U.K., it is known as a "share option".

American options can be exercised anytime between the date of purchase and the expiration date. European options may only be redeemed at the expiration date. Most exchange-traded stock options are American.

Source: http://investopedia.com/terms/s/stockoption.asp