Trading a single security on two different exchanges. For example, one may buy a stock on the American Stock Exchange and sell it on the NYSE. Intermarket trading is a fairly common arbitrage strategy, whereby one takes advantage of the difference in price on the same security on two different exchanges in order to make a profit.
Trading that occurs between two or more markets. For example, an individual might engage in arbitrage by purchasing stock index futures on one exchange and then selling options on the same futures on another exchange.