Short
A position created by selling an option to open, obligating you to fulfill the contract if assigned.
Explanation
Short options positions collect premium upfront and benefit from time decay and decreasing volatility. Short calls obligate you to sell the underlying; short puts obligate you to buy. Short options have negative theta (benefit from decay) but carry assignment risk. Short option sellers want the option to expire worthless.
Example
Selling (going short) a 150 call on AAPL for $3.00 means you collect $300 and accept the obligation to sell 100 shares at $150 if assigned.