Katrina,
I would like to thank you for giving my inner geek an opportunity to fully express himself...
So here's the deal: The word both Matthew and Luke use is the word in the temptation narrative is "diabolos" (from which you can easily discern the Latin, Spanish, etc.). The noun diabolos is derived from the verb "diaballo," which itself is a compound word from the preposition "dia" (= through) and "ballo" (= to throw). Pretty exciting, eh????
Diabollo (verb) literally means "to throw across" or "to divide, separate from," but used in the sense of "to oppose" or "to accuse." Can also be used in the context of deception (e.g., "to misrepresent").
The reason one cares that the noun is derived from the verb is that it's a pretty good indication that the noun is meant to objectify the qualities of the verb. That is to say, there isn't a "thing" that gets named by the noun. Rather, there is a quality of being that - when expressed - is denoted by the noun. In short: there is no "thing" out there called a "devil" that you can put in the zoo. Rather, there is a thing that has the qualities of opposing, separating, speaking falsehood that can be seen as having substance. So you are very much correct with your suggestion that the label is being applied to something that cannot be otherwise understood. In the temptation story, "devil" is something of a placeholder for whatever it was that tempted Jesus.
If we choose to get even more geeky (and why wouldn't we????), we can examine how the word evolved over time (there is a really great set of reference books that is very helpful with this - not like it's something carry in my brain; I'm not THAT geeky!...please....).
First place we'd want to look is Classic Greek (the stuff Plato, Aristotle and the like used - no Xn influence). There the term is very rare. Good to know.
Next place to look is the Septuagint (the Gk. translation of the Hebrew OT - known in shorthand as the LXX; it's the "Bible" most in Jesus' day would have read).
In the LXX diabolos is used to translate - you guessed it - the Hebrew word "satan" (which means..."opponent, adversary..."). Diabolos is found a grand total of 21 times in the LXX, 13 of them in Job 1-2 if that helps. But - before we get too comfortable - it is also used in places like 1 Sam. 29:4 where it is used to describe King David (clearly not a spiritual being in opposition to God; Hebrew word here is satan; the context of this passage is important in that David is being regarded as an adversary by the Philistines, but not one that they worry too much about - a not-so-threatening opponent).
Side note: Interesting that Matthew (writing to a very Jewish audience) uses the word diabolos, whereas Mark (writing to a very Gentile audience) uses the word "satan."
If we were writing our PhD dissertation on this (and that would be rad!), we would focus on the inter-testamental period which is where the idea of "devil" and "Satan" (as a "person") become vastly more important figures. It is a fascinating study from an historical perspective, but a little more difficult on the religious front in that the ideas that develop in late-2nd Temple Judaism (say 200 BC - 70 AD) depart a bit from the OT. What Israel does with the concept of Satan in say 50 BC does not line up very well with the theology of the OT.
We see this especially in the theology of the Essenes. In the Dead Sea Scrolls they describe God as having created two spirits - an Angel of Light and an Angel of Darkness who were fighting it out for power in this age. Those who were pure (i.e., the Essenes) were Sons of Light. Those who were apostate or enemies of righteousness were Sons of Darkness. BTW, the name of this Angel of Darkness was Belial.
So there's the short version of the basic data.... And the key questions for NT scholars everywhere is: "So what are the Gospel writers trying to say when they use terms like 'satan' and 'devil'?"
No obvious answer. But the really cool thing is that by entering into the debate, we step into the world of the great Christian thinkers - past and present. When we wrestle with these things, we are wresting alongside Augustine, Aquinas, Luther, Calvin, Barth... They did not occupy themselves with the easy questions, but with the hard ones. If we struggled with the things for which there are simple explanations, we would not be very mature. That we can take in all the data and accept ambiguity as part of the life of faith, we step into the realm where true maturity can happen. And I think that is really cool! :-)