[Thalion]

This should settle it:

Letters #131: To Milton Waldman:

[Tolkien is speaking about the end of the 1st age and how the history of middle earth plays out]

"Also the Orcs (goblins) and other monsters bred by the First Enemy are not wholly destroyed."

We can gather from this sentence that Tolkien considered the terms interchangable? to a degree as he clearly puts goblins in paraenthesis after Orcs...however, we also know that he was more fond of the term Orc rather than goblin, but none-the-less...they are the same thing...

Additionally, and more importantly, Letters #144: To Naomi Mitchison:

"Orcs (the word is as far as I am concerned actually derived from Old English orc 'demon', but only because of its phonetic suitability) are nowhere clearly stated to be of any particular origin. But since they are servants of the Dark Power, and later of Sauron, neither of whom could, or would, produce living things, they must be 'corruptions'. They are not based on direct experience of mine; but owe, I suppose, a good deal to the goblin tradition (goblin is used asa translation in The Hobbit , where orc only occurs once, I think), especially as it appears in George MacDonald, except for the soft feet which I never believed in. The name has the form orch (pl. yrch) in Sindarin and uruk in the Black Speech."

Again, Tolkien himself uses the words interchangable, specifically saying that the translation used in Hobbit was mostly goblin, but once orc...the same creature