A scalp in trading is the act of opening and then closing a position very quickly, in the hope of profiting from small price movements.
Traders who practise this tactic are referred to as scalpers, and will tend to make many scalps each day. The theory behind scalping is that small price movements are easier to predict than large ones.
Profits on scalps tend to be small, but losses can be kept to a minimum as strict rules are adhered to.