Discount Arbitrage

Discount arbitrage is a trading strategy that tries to lock in a near risk-free profit when an option is trading for less than its true, calculable worth. An option is a contract that gives its buyer the right (not the obligation) to buy or sell a stock at a set price, called the strike price, by a certain date. Every option has a theoretical fair value based on things like the stock's price, the strike, time left, and volatility. When an option's market price falls below what that math says it should be worth, traders call it a discount, and some traders build a position designed to capture that gap.

The mechanics work by pairing the underpriced option with an opposite position in the underlying stock, so that the two positions offset each other's directional risk. In basic call arbitrage, the trader buys a call (the right to buy stock) at a discount and simultaneously sells the same amount of the underlying stock short. In basic put arbitrage, the trader buys a put (the right to sell stock) at a discount and simultaneously buys the underlying stock. In both cases, if the stock moves, the gain or loss on the stock position is meant to be cancelled out by the opposite move in the option's value, leaving the trader holding just the pricing discrepancy as profit once the position is closed or the option is exercised.

The nuance that trips people up is the word "riskless." This strategy is only close to riskless in a frictionless world with no transaction costs, no early-assignment surprises, no changes in dividends or interest rates, and instant, guaranteed execution at the prices used in the calculation. In real markets, the discount can be tiny, commissions and the bid-ask spread can eat it entirely, and by the time a retail trader spots and executes the trade, the mispricing has often already vanished because faster, automated traders closed it first.

It's also worth separating this from ordinary discount language, like an option simply being "cheap" for a reason (low liquidity, wide spreads, or an incoming event). Discount arbitrage specifically refers to exploiting a measurable deviation from fair value using a hedged, offsetting position, not just buying something because it looks inexpensive.