Gamma

If delta is speed, gamma is acceleration. An option with high gamma sees its delta swing sharply on small moves in Nifty or Bank Nifty, which makes the position behave very differently within minutes. Gamma is highest for at-the-money options and rises steeply as expiry nears.

This is why weekly expiry day on the NSE is dangerous for option sellers: at-the-money options carry enormous gamma, so a small index move can flip a comfortable position into a large loss almost instantly. Option buyers love this gamma; sellers fear it and often square off before the final hours.