It's easy in designing a game, particularly an RPG, to overlook the importance of building a good vocabulary for your game. It's not a particularly exciting or thrilling part of the process, but if you screw it up you can turn your game into an annoying, unplayable mess.
When you slap a label on a mechanic you're creating your game's jargon. This applies specifically to measures like hit points, tools like skills and weapons, and anything else that the people sitting at the table, playing your game, need to refer to.
My rule of thumb, after working on a number of games and seeing terms soar or flop, is that similar mechanics need very different names, particularly mechanics that run into each other alot.
In 4e, temporary hit points are a poster child for this. They look and act a lot like hit points, thus the shared name, but if you treat them like hit points the game goes haywire. The problem is that, unless someone reads the rules in depth, it's very easy to overlook that word "temporary" in temporary hit points. With a similar name and 90% similar mechanics, it's easy to mesh the two together.
On the other hand, a shared name is useful as long as you apply it to a mechanic or measure that uses it correctly. In D&D, there are 10,000 things that are given a level, but it's rare (despite "Use of the Term Level" headers in early versions of D&D) for people to confuse the term. Magic items, monsters, characters, dungeons, and so on all use level the same way: higher level means more powerful. In that case, a single term keeps things simple and clear. It helps that it's hard to figure out a way to conflate a level in a dungeon with an item's level. The two don't interact in a meaningful way!
So, those are my brief thoughts on naming mechanics in your games.
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