Traditionally, pirates are raiders who prowled the seas of Earth, hijacking merchant shipping. By the 24th century, space piracy was widespread enough for the UNSC to adopt the UNSC Convention on Admiralty Law of 2315. Groups such as the Eridanus Rebels based themselves in the system's asteroid belt, using it as a base of operations for their activities until Operation: TREBUCHET. It is unknown if other groups used similar tactics.
Space piracy was not restricted to humanity alone. The Kig-Yar had a long seafaring history on their homeworld, and when they became a space faring race, piracy flourished in their system as pirate factions raided merchant shipping. When the Covenant discovered the Kig-Yar, these pirate bands became the only effective fleet the species could field in self-defense. In order to survive, these same pirates became privateers in the service of the Ministry of Tranquility. In theory, these privateers patrol Covenant space in an exploratory mission. In practice, it is little more than an excuse for the Kig-Yar to salvage whatever they can. Piracy remained heavily ingrained in Kig-Yar culture, as all their great folk heroes were famous pirates, representing freedom to those Kig-Yar in the service of the Covenant. Each Kig-Yar chick hoped to be a pirate when they grew up.