When used in trading, long refers to a position that makes profit if an asset’s market price increases. Usually used in context as ‘taking a long position’, or ‘going long’.
Going long is the opposite of going short or shorting, which means taking a position that makes a profit if an asset’s market price falls.
Taking a long position doesn’t necessarily mean buying an asset. Derivatives like spread bets, CFDs and futures contracts all provide the facility for traders to take a long position on a market without actually buying the underlying asset.