Hover your mouse over a word to see it's definition (it saves a lot of page space, honestly).
pirate
The basic terminology. Going back pretty much as far as there has been sea travel, and continuing today, a pirate is basically a highway robber of the high seas, someone who attacked merchant or other vessels to steal supplies and riches, and maybe take hostages. This was often done for personal gain, sometimes to get out of debt or make a fortune so one could then retire into a life of leisure with a new identity. Others were acting on principles or waging terrorism against governments or classes of people they disliked. Buccaneer is a synonym and mostly there's no difference between the two phrases, especially during the Golden Age of Piracy or discussions of it, but note the difference between these two terms and a privateer.
buccaneer
Also a football team in Tampa, or a dad joke about the cost of corn.
privateer
Similar to a mercenary, this was a pirate who was granted carte blanche (for the most part) by a government or established commercial interest to attack and plunder only certain ships, usually those of political or commercial rivals, and keep most of what they stole, minus a cut for the guarantor. This was actually a whole political strategy during times of war or the threat of it, and not only terrorized an enemy (including their civilians) or robbed them of capital, ships, and men, but was also an important means of enriching the guarantor. For more on privateering, note the page linked to from the "Piracy" page.
galleon
A huge Spanish ship that was sometimes used as a warship, but due to being so huge and ungainly, was mostly used as a cargo freighter. These slow-moving wooden vessels, when weighted down with goods, were obviously vulnerable targets for pirates, and so often travelled in fleets, escorted by smaller, more nimble warships such as sloops.
pieces of eight
A Spanish monetary unit of silver, equivalent to somewhat less than 1 US dollar in 2022. Like the American dollar is today, it was accepted in foreign lands and used as the standard by which other currencies were set, and remained legal tender (that is, accepted currency) in the US from colonial times until 1857. It was actually very influential on forms of currency around the world from the time of Columbus to today. This is an interesting page on it, and the Wikipedia page on it is also pretty cool.
sea-dog
Pirates probably didn't actually call people this all the time--"pirate-speak" is mostly an invention of 19th-century adventure novels propagated by 20th-century swashbuckling movies--but a sea-dog was actually what English-speaking sailors called sharks prior to contact with "the New World", possibly due to the similarity in appearance between a shark feeding frenzy and similar behavior by packs of dogs. The word "shark" is an Anglicization of xooc, an Aztec word that would have sounded similar to "shock" (that x makes a "sh" sound) and the Spaniards adopted the word while committing genocide and plundering their new finds as conquistadors, and the word would have circulated among sailors around the world during the age of European imperialist expansion in the following centuries. Calling someone "a scurvy sea-dog" is basically a child-safe way of a fictional bad guy insulting someone in a swashbuckling story; real pirates probably would have just cursed you out just like modern criminals do.
scurvy
This was a disease that was a real problem in sailors in times before a modern understanding of health and nutrition. Due to a severe deficit of vitamin C, people who developed scurvy suffered excruciating and disgusting deaths due to the connective tissues in their bodies disintegrating. Fingernails and teeth would fall out, long-healed old scars would open up, and broken bones healed in childhood would re-break along their old break lines. Then you died. It was gross. Some ship captains noticed a correlation between giving their sailors citrus to eat and a lack of scurvy among the crew, so in xxxx a doctor of the English navy literally prescribed that barrels of oranges, lemons, or limes be carried aboard their ships and that sailors be encouraged to eat them. Eventually, European science figured out what vitamins are and how they work, explaining why sailors who had access to fresh citrus were able to ward off scurvy. It was common for English sailors in port towns to be seen doing their work or exploring the town while sucking on a lime--just like how you used to pretend oranges were mouthguards when you were a kid--and since Liverpool has always been a port town, the English sailors that were a constant presence passing in and out were nicknamed "limeys", a term that eventually started to include locals of the area as well.