privateer n
an armed ship in the past that was not in the navy but attacked and robbed enemy ships carrying goods [LDCE]

buccaneer n
a pirate, esp one who preyed on the Spanish colonies and shipping in America and the Caribbean in the 17th and 18th centuries [CED]

“What’s a privateer?” Steve asked, puzzled. “Some kind of buccaneer?”

“Sort of.” Rita laughed. “The line was always blurry. Basically, a government that was at war would commission pirates to fight for them against the enemy. In peace, pirates might attack any ship they wanted. The trouble is, the term turns on a legal technicality. They were basically the same people, doing the same things.”

William F. Wu, Isaac Asimov’s Robots in Time 2: Marauder, 1993

From 1660 until 1720, the so-called golden age of piracy, pirates again operated as privateers. This period saw some sailing under the famous “Jolly Roger” flag, with attacks by English pirates on both Spanish and French ships. There were also English attacks on the Dutch; the island of Saint Eustatius, a Dutch sugar island, was attacked by pirates and British soldiers on many occasions, changing hands 10 times during the 1660s and early 1670s. French pirates also started operating freely from their ports on the island of Hispaniola (modern-day Haiti and the Dominican Republic). Sir Henry Morgan, a Welsh buccaneer, sacked the Spanish town of Portobelo in Panama, which had been well garrisoned.

Justin Corfield, “Piracy in the Atlantic World”, in Encyclopedia of World History, vol. 3, Facts On Files, p. 309