A bell that rings to signify the end of a trading session. The closing bell occurs at 4:00 pm EST. Between 1870 and 1903, a gong was used. A bell was then introduced and is still in use today. Not all exchanges use this traditional system - the New York Stock Exchange is one that does.
Taobiz explains Closing Bell
NYSE began having special guests ring the closing bell on a regular basis in 1995. This daily tradition is highly publicized and often done by a company. Prior to 1995, ringing the bell was usually the responsibility of the exchange's floor managers. There are bells located in each of the four main sections of the NYSE that all ring at the same time once a button is pressed. The ringers press the button for approximately 10 seconds, and a gavel sitting at the front is also used on some occasions.