Piracy is an illegal act typically involving violence or the seizure of property carried out against a watercraft, aircraft, spacecraft, or port. By extension, acts of participating in or inciting such activities are defined as piracy by the UNSC Convention on Admiralty Law of 2315.
Traditionally, pirates are remembered as raiders who prowled the seas of Earth, hijacking merchant shipping. But the important point is that they did it on the water - when humanity broke free of the constraints of Earth's atmosphere, it was almost inevitable that space piracy would become the new lucrative threat to shipping. By the 24th century, space piracy was widespread enough for the UNSC to adopt the UNSC Convention on Admiralty Law of 2315, which authorizes the summary execution of pirates by the forces of an officially recognized government. The budding threat of piracy in space also contributed to the creation of the Colonial Military Administration in 2310, one of its main functions being the protection of colonial vessels from raiders as humanity began interstellar expansion. This laid the foundation for the Unified Earth Government's military presence in the Outer Colonies; the very involvement which would later provoke the colonists' ire and contribute to the sparking of the Insurrection.
The Insurrectionist movement was frequently engaged in pirate activities, and by 2517, pirates were considered a serious threat to vessels traveling in the Outer Colonies without an armed escort. Groups such as the Eridanus rebels based themselves in the system's asteroid belt—most prominently the habitat known as Eridanus Secundus—using it as a base of operations for their activities throughout the Insurrection and well into the Human-Covenant War.
The crew of the Ace of Spades were pirates to an extent.
Space piracy is 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 joined Eayn's government's militia to fend off the Covenant intruders. In order to survive, these same pirates became privateers in the service of the Ministry of Tranquility. In theory, these privateers patrolled Covenant space on an exploratory mission. In practice, it was little more than an excuse for the Kig-Yar to salvage whatever they could. 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 hopes to be a pirate when they grow up.