More fantasy-focused this time.
ravish/ravage. No, not the same word, sorry to disappoint you. "Ravish" means to rape or to enchant (contradictory meanings, I know), since it has a root meaning of "seize, carry off." Ravage means "to destroy," and is usually the word that people want, since it can be used in the context of, say, Viking look-alikes hitting and destroying a village. But it makes no sense to say, "Ravage the women and ravish the village!"
Ravish does have a place in fantasy, as a means of speaking about rape without necessarily introducing a too-modern vocabulary. But please use it in the right way, since used wrongly it will make the knowledgeable part of your audience roll their eyes (or snort and giggle, or throw the book across the room).
disinterested/uninterested. This distinction is blurring in modern English, but it's one worth keeping in mind when you're writing fantasy, which generally uses a more correct and formal vocabulary than other genres. It can also get you in trouble with a knowledgeable audience, since the old meanings are separate.
Uninterested is the meaning that most people want, since it means a simple position of neutrality or ignorance. Disinterested indicates impartiality, of the kind a judge should exhibit, and a cool, academic distance from the problem that's not the same as just choosing to ignore it; the disinterested person is willing to criticize, which the simply uninterested person probably isn't. They're not hard to employ and use in separate contexts, once you know what they mean.
(Damn blurry English).
sentient/sapient. Some people try to use sapient to get around the problem of meaning in sentient, which supposedly only means "feeling," and is therefore an incorrect word to call an animal-like fantasy race that also has intelligence. However, sentient can also mean conscious, and there's probably enough leeway in there to slip around the problem. This is especially needed because sapient doesn't just mean being conscious; it means keenly perceptive and wise. There has to be middle ground somewhere in between an animal-like brain and intense wisdom, and it's probably best to use "sentient" to cover that ground.
carbuncle. Do be careful before you use this one. It does mean a red precious stone, especially a garnet or ruby sapphire, but it also means a nasty infection that leads to patients experiencing sloughing of the skin, depression, and most likely death. Its other name is anthrax. So when you're talking on about the precious carbuncle of the Sun Kingdom, consider if there's another word for a red gem you could use first.
amber. Most people already know this, but it's worth a reminder, since I've seen a lot of stories lately that talked about amber "mines": Amber is the fossilized sap of a tree, not a rock. If you have a country that produces a lot of amber in your fantasy series, there should be old forests around, not mountains.
electrum. This is nothing wildly special. It's either another name for amber, or the alloy of silver and gold. If you have electrum coins in your fantasy series, fine, but you have to have the means of forging gold and silver together or getting amber first. Electrum's name comes from the Greek for electricity, but it doesn't attract it particularly.
claustrophobia. Along with all the other phobias, this term is something you should be careful about using. They're familiar in modern culture as medical terms or as convenient excuses for problems, whether or not they're actually medically diagnosed. But consider how this term sounds in a fantasy world. Is it medically advanced enough to distinguish between different kinds of fears? More than that, is it likely that your character could afford to see a healer or the equivalent and pay to have his or her fear diagnosed, or that he or she would ever have the chance? I keep reading about characters who seem to have been diagnosed thoroughly by at least the local psychoanalyst, even though they've spent all their lives running around in the woods chasing monsters.
If your character has a fear of closed spaces, that's fine. I would just think twice before calling it claustrophobia.
archenemy/nemesis. These have different meanings. "Archenemy" just means that someone is your character's worst enemy; it says nothing about why he's pursuing the character or whether he can be beaten. "Nemesis" means either someone who's pursuing your character because he wants justice or revenge, or someone who cannot be defeated, which is the reason that someone can also speak of his characteristics as nemeses.
"Nemesis" sounds more dramatic and cool, but do consider what you're saying with it. Is your character doomed to never escape this enemy? Is the enemy after him because he killed the enemy's family, say? I think the distinction worth preserving, because it will tell your readers how seriously they should regard this enemy.
vengeance/revenge. These are very similar in meaning, but again have distinctions. Revenge is the less justice-like, the more violent, and the more often committed for a perceived wrong than for a real one. Vengeance is usually seen as more justified or justifiable. It will, again, tell your readers how they should regard this if your character is seeking vengeance or is seeking revenge.