A number of terms, mostly denoted by Greek letters, that measure an option’s (or a position’s) sensitivity to the price of the underlying security, time decay, and volatility. They are delta, gamma, theta, and vega. Amusingly, vega is not a Greek letter, but is still lumped in with “the Greeks.” These quantities are used by traders to construct portfolios that react in certain ways to changes in time, volatility, or price of the underlying security.