The economic word for today is arbitrage.
In economics, arbitrage is the practice of taking advantage of a state of imbalance between two (or possibly more) markets: a combination of matching deals are struck that exploit the imbalance, the profit being the difference between the market prices. A person who engages in arbitrage is called an arbitrageur.Basically, arbitrage is taking advantage of the common economic advice to "buy low and sell high". You find two markets trading in the same thing (stock, commodity, currency, whatever) and buy from the market with the low price and sell to the market with a high price. The trick is often in identifying the item to trade and the markets in which to do the trading so that the trades are profitable.
Arbitrage is possible when one of three conditions is not met:
1. The same asset must trade at the same price on all markets ("the law of one price").
2. Two assets with identical cash flows must trade at the same price.
3. An asset with a known price in the future, must today trade at its future price discounted at the risk free rate.
Due to human psychology, some markets are in permanent disequilibrium. Consider, for example, the endless spending on weight-loss.
Also of interest: bookmaking.