A speculator on the trading floor of an exchange who buys and sells rapidly, with small profits or losses, holding his positions for only a short time during a trading session. Typically, a scalper will stand ready to buy at a fraction below the last transaction price and to sell at a fraction above, e.g., to buy at the bid and sell at the offer or ask price, with the intent of capturing the spread between the two, thus creating market liquidity. See Day Trader, Position Trader.

Source: Commodity Futures Trading Commission