Long is explained together with its opposite under long and short positions, which covers how each kind of position makes or loses money and how traders use them. This page explains the single word as it is used in markets.
To be "long" an asset means to own it, or to hold a contract that gains in value when its price rises. An investor who buys shares, a commodity or a cryptocurrency outright is long that asset. A trader can also go long through derivatives, for example by buying a futures contract or a call option, or by opening a long position on perpetual futures on a crypto exchange. The word works as an adjective ("long bitcoin"), a noun ("a long") and part of phrases such as "long-only fund".
The opposite is to be short, which profits when prices fall, usually through short selling. Being long reflects a bullish view, although many long positions are held simply as investments with no short-term view at all. The maximum loss on an unleveraged long is the amount paid, while the potential gain is in principle unlimited. Traders sometimes hold both long and short positions at once as a form of hedging.