From Traderpedia

A position in which the trader/investor benefits from appreciation in the price or value of the security in question.

Traditionally long is thought of in terms of ownership, such as owning (being long) 500 shares. However, one is also considered to be long in the case where one will be the purchaser in the future. An example of this would be a futures contract in which one is to be the purchaser at delivery. One who is long a gold futures contract would be the trader expecting to take delivery of the gold. As such an increase in the price of gold would benefit the trader.

This is the opposite of being short.