Pytheas of Massilia (Marseille) visited Britain - probably sometime between 322 and 285 BC - and described it as triangular in shape, with a northern tip called Orcas. This may have referred to Dunnet Head, from which Orkney is visible. Writing in the 1st century AD, the Roman geographer Pomponius Mela called the islands Orcades, as did Tacitus in AD 98, claiming that his father-in-law Agricola had "discovered and subjugated the Orcades hitherto unknown" (although both Mela and Pliny had previously referred to the islands.) Etymologists usually interpret the element orc- as a Pictish tribal name meaning "young pig" or "young boar". Speakers of Old Irish referred to the islands as Insi Orc ("island of the pigs"). The archipelago is known as Arcaibh in modern Scottish Gaelic, the -aibh representing a fossilized prepositional case ending.

Norwegian settlers arriving from the late 9th century re-interpreted orc as Old Norse orkn "seal", with the added suffix ey "island". Thus the name became Orkneyjar (meaning "seal islands"), later shortened to "Orkney" in English. According to the Historia Norvegiæ, Orkney was named after an earl called Orkan.

The Norse knew Mainland Orkney as Megenland (mainland) or as Hrossey (horse island). The island is sometimes referred to as Pomona (or Pomonia), a name that stems from a sixteenth-century mis-translation by George Buchanan and has rarely been used locally.