An approximation of the change in the delta of an option relative to a change in the price of the underlying stock when all other factors are held constant. Gamma is accurate for small changes in the price of the underlying stock, but is expressed in terms of a change in delta for a 1 point move in the stock. For example, if a call has a delta of .49 and a gamma of .03, if the stock moves down 1 point, the call delta would be .46 (.49 + (.03 x -$1.00)). Generated by a mathematical model, delta depends on the stock price, strike price, volatility, interest rates, dividends, and time to expiration.
A type of limit order that is active until it is filled or canceled. As opposed to a day order, a GTC order can remain active for up to six months.
Regarding options, it's a colloquial term for the analytic measurements such as delta, gamma, theta, vega and rho, etc.