Limits to Arbitrage Definition
Limits to arbitrage is a theory which assumes that restrictions placed upon funds, that would ordinarily be used by rational traders to arbitrage away pricing inefficiencies, leave prices in a non-equilibrium state for protracted periods of time.
The efficient market hypothesis assumes that whenever mispricing of a publicly-traded stock occurs as a result of an over-reaction to news, or some similar event, an opportuntity for low-risk profit is created for rational traders. The low-risk profit opportunity exists through the tool of arbitrage, which, briefly, is buying and selling differently priced items of the same value, and pocketing the difference. If a stock falls away from its equilibrium price (let us say it becomes undervalued) due to irrational trading (noise traders), rational investors will (in this case) take a long position while going short on a proxy security, or another stock with similar characteristics.