Conversely, I can easily use the word "unoriginal", but try to be sensible, empathic and plausible while doing that. As, my experiences proved long ago that some "similarities", and even virtually identical productions can be the results of sheer coincidences.

Fun trivia: People liken me (slightly physically, but more in terms of character, temperament, and way of thinking and talking) to Sheldon Cooper of The Big Bang Theory, and I swear that I haven't watched even an episode of that TV series. (I had to do a Google search to write the name here.) A friend of mine gave me the link of a YouTube video composed of its best scenes where Sheldon Cooper acts, only last year. I still am not interested, haha.

Anyway, I believe all of us can give countless similar examples, but my actual point is that; in order to be sure of whether something is unoriginal or not, we also have to be sure that the creator of an output -which can be deemed as unoriginal- is/was aware or unaware of the "original" thing that s/he is accused of ripping off.  This shouldn't necessarily mean that we cannot "accuse" anybody of this. For instance, if a metal band do a very similar song to Metallica's one big hit, they cannot/shouldn't easily get away with that... Come on... You must be aware that such a song exists!

Any thoughts/insights/arguments?

Very easy. Originality is something that hasn't been done before. The magnitude of which can vary greatly.

The problem with defining something as original lies in the fact that evolutionary processes are gradual incremental developments.

Occasionally a big bang of creativity can occur but more often than not it is a result of subtle changes of what has been done before.

So what's the point?

I would agree that I would expect a metal band to have heard this one big hit, but suppose a band of another genre created a song using a similar riff and claimed they had never listened to Metallica - how would you prove otherwise?

I sense, implicit to “originality” as it’s being used in this thread, the idea of an isolated artwork or idea, having nothing to do with what has come before. I think that’s sort of the artistic equivalent to the idea of ‘spontaneous generation’ in the realm of science; in other words, loaded thinking without basis in reality.

I looked up 'Original' in the dictionary and this is what I saw...