Long / short
Long means owning a position that was bought. Short means having sold a position not owned, such as a written option, which carries an obligation.
A long option holder paid the premium and holds the right that comes with the contract. A short option holder, often called the writer, received the premium and holds the obligation to buy or sell the underlying if assigned.
Long options have a loss limited to the premium paid, while a short call that is not covered by stock has losses that can grow as the underlying rises, and a short put’s loss can reach the strike minus the premium received. The same words apply to shares: a long stock position owns the shares, and a short stock position has borrowed and sold them.
Illustrative example (hypothetical numbers, not a real trade): one trader buys a $20 call for $1.00 and is long the call. The trader on the other side sold it for $1.00 and is short the call. If the stock is at $23 at expiration, the call is worth $3.00, so the long side has gained $200 and the short side has lost $200 per contract.