Orc

orc, ork, Orc, Ork, Yrch, n., Orcish , adj. 1. a sea-monster fabled by Ariosto, Drayton, and Sylvester to devour men and women; according to Pliny, it was a huge creature 'armed with teeth.' 2. in the works of J.R.R. Tolkien, a race of malevolent creatures descended from Elves corrupted by Morgoth; a.k.a. Orch, Yrch, Glamhoth, Gorgûn, Goblin, Hobgoblin, Uruk. 3. in fantasy literature and RPGs, a race of malevolent creatures inspired by Tolkien's orcs, usually dark and stunted troll-like creatures with pointy ears, tusks, and bad attitudes. See also bugbear. [GJ1]
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Addendum: It's unclear why Tolkien named his goblin creatures after a mythical sea monster, except that Tolkien often borrowed mythological names and changed their meaning. For instance, in the Eddic Sagas, Gandalf is the name of a dwarf. Also, note that while Tolkien identified Orcs and Goblins as the same creature, in many fantasy milieux such as D&D, they are different monsters. And finally, many artists depict orcs, both Tolkien's and others, as some kind of "pig-men." There is no basis for this, esp. in Tolkien (who could describe and identify 50,000 types of trees, but never clearly described what orcs look like). Peter Jackson's version (see below), depicting orcs as something like mouldering elf-corpses, seems more appropriate. [GJ1]
Addendum #2: As most any gamer knows, the goblins of [[hobbit} The Hobbit]] became the orcs of LOTR. "Orc" is used in The Hobbit once, just after Bilbo escapes from Gollum; the tunnel along which he runs is described as being too big for goblins "not knowing that even the big ones, the orcs of the mountains, ...." It is clear (to me) that Tolkien was using orc as a simile (or is it metaphor?) for meaning just a giant goblin, comparing it to the giant sea creature mentioned in Ariosto, for instance. I tend to believe that orc and Orcus (not the demon, but the whale) are from the same root, suggesting large size. Possibly when he was writing LOTR he wanted to dispense with goblin, as it suggests something more appropriate to a juvenile fantasy, rather than the vicious orcs as they were in LOTR, and seized upon a word he had already used. [CP]
Addendum #3: As for the etymology of orc, I submit this, from Author of the Century by Tom Shippey, who, like Tolkien, is a linguist: "Tolkien preferred an Old English word [over 'goblin'], and found it in two compounds, the plural form orc-neas found in Beowulf [which Tolkien translated early in his career], where it seems to mean "demon-corpses," and the singular orc-thyrs, where the second half is found also in Old Norse and means something like "giant." Demons, giants, zombies - it seems that literate Anglo-Saxons really had very little idea what orcs were at all. The word was floating freely, with ominous suggestions but no clear referent. Tolkien took the word, brought the concept into clear focus in detailed scenes and has in a way made both word and thing canonical." [GJ1]
Addendum #4: But wait, there's more! According to The Languages of Middle Earth by Ruth S. Noel, OE orc "demon" is derived from Latin Orcus "God of the Dead." See Wand of Orcus. [GJ1]