A person or company who undertakes a set of transactions involving buying in one market and selling in another, where the prices are known simultaneously. Thus, although a profit can be made if the prices are different, an arbitrageur takes very little risk. The term arbitrageur is also used to describe those who buy and sell companies or parts of companies at pre-arranged prices, again taking very little risk. An arbitrageur is contrasted with a speculator, who buys and sells in markets where the prices are not known simultaneously, so that between purchase and sale the speculator is at risk.