Capping

Capping is when selling pressure is placed on a stock (or another asset) to either lower the price or keep the price low.

Capping is a strategy to game the market when someone has written a call option on an asset they are otherwise long on. Say you hold a lot of a stock (that's what it means to "be long"). You write a call option on that stock, meaning someone else pays you for the right to buy the stock at a pre-arranged price. The call option has an expiration date, so if the stock stays below the call's strike price by the expiration date, the option will end up worthless.

In capping, the person who wrote the option will sell a sizable amount of the stock they own in order to create downward pressure on the stock. They are thus capping its price. If it works out, the option will expire without getting exercised.