A machine in which a coin or token is dropped into a slot and a lever or button pulled to activate one to three or more reels marked into horizontal segments by varying symbols; the spinner may line up two, five, or all coins to make a payout. In early machines, coins fell onto an internal balance scale and were deposited into a cup or trough; later devices used a spinning indicator that came to rest on or pointed to a number, suit, symbol, or picture. Forces of morality and the clergy, as well as city ordinances, often opposed the operation of slot machines. Invented in 1888 by Charles Fey of San Francisco, the first commercially successful slot machines were electromechanical and allowed different payout schemes: the size of the winnings depended on the number of coins or tokens that lined up when the spinning indicator came to rest; the symbols varied from slot to slot, but usually included stars, card suits, bars, sevens, and various pictured fruits-cherries, oranges, lemons, and watermelons are common.

Since microprocessors were applied to classic three-reel steppers, slot developers and operators have sought to understand what features appeal to players. Firms such as ReelMetrics help casinos and suppliers develop games that entice players to stay longer and play more. Other companies, such as SG Digital, have launched reporting systems that provide game designers with valuable data on player behavior. These methods, unlike those of psychophysiological measures that require cumbersome electrodes and wires to collect data (which reduce ecological validity and may inhibit flow), are unobtrusive, relying on the observation of actual play in a casino setting.