Five numbers, one option
Every option price responds to five inputs, and each response has a name. Delta measures how much the price moves per dollar move in the underlying — calls run from 0 to 1, puts from −1 to 0, and either can be read as a rough odds of finishing in the money. Gamma measures how fast delta itself changes — the curvature behind delta, and the number that decides how urgently a dealer has to keep adjusting a hedge.
Theta is time decay: dollars an option loses per day from the calendar alone, accelerating as expiration closes in and fastest for at-the-money contracts in their final days. Vega measures sensitivity to implied volatility — a one-point move in IV changes the option's value by roughly its vega, and a long option is a long-vega position that benefits when volatility rises. Rho, sensitivity to interest rates, is usually the smallest of the five for short-dated equity options, and matters more the further out an option expires.
Where the strike sits
Moneyness describes the strike's position relative to spot. In-the-money means the option would carry intrinsic value if exercised right now — a call struck below spot, or a put struck above it. At-the-money means the strike sits close to spot, which is also where gamma and theta both run hottest.
Out-of-the-money means the strike carries no intrinsic value yet, only time value — the option is a bet that the underlying gets there before expiration. Every exposure metric described elsewhere in this library is built strike by strike, and moneyness is the first filter for reading any of them: the walls that matter most tend to sit near the money, exactly where gamma is largest.
The scaffolding underneath
Open interest is the number of contracts still outstanding at a strike and expiry — not yet closed, not yet exercised. It is the "how much" behind every exposure metric on this site: gamma exposure, delta exposure, and max pain are all built by weighting each strike's Greeks by its open interest.
The forward price is the risk-neutral price the options market is actually pricing around — roughly spot grown at the risk-free rate, net of dividends — not simply today's ticker. Gamma scalping is the mechanical habit behind one of this library's central ideas: a trader or dealer holding long gamma continuously rebalances the hedge as price moves, capturing a small profit on each rebalance. It is the reason positive-gamma regimes compress range, the subject of the next concept.