Another pedantic rant (brought to my attention by my erudite sister Chandra). So often people see something gross, start feeling sick, and say, "Oh my, I feel nauseous!"

When one feels like vomiting, one feels NAUSEATED. When something causes nausea, that thing is said to be NAUSEOUS. The American Heritage English Dictionary sez:

nauseous Causing nausea; sickening nauseated To be feeling, or having been caused to feel nausea.

So, next time you are tempted to say "I feel nauseous", understand that you are saying "I feel that I make other people sick", or basically "I feel nauseating".

Not that it's never correct to say "I feel nauseous". For example, if you fell in a pile of human entrails and got up with various bits of human anatomy draped around your body, you would probably be correct in calling yourself nauseous. (But you'd probably also be nauseated, and I suspect your pending loss of lunch would still be the emotion you'd want to describe instead of caring how you made other people feel.)

In writing this rant, I looked up the actual definitions above, and in doing so found this Usage Note:

Traditional critics have insisted that nauseous is appropriately used only to mean "causing nausea" and that it is incorrect to use it to mean "affected with nausea," as in Roller coasters make me nauseous. In this example, nauseated is preferred by 72 percent of the Usage Panel. What is curious, however, is that 88 percent of the Panelists indicated that they would prefer nauseating in the sentence The children looked a little green from too many candy apples and nauseous rides. Thus it appears that like a handful of other words such as transpire, nauseous is actively used mainly in the sense in which it is considered incorrect. While the use of nauseous to mean "affected with nausea" may incur critical displeasure, it should be pointed out in its defense not only that it is quite common among educated speakers but that it is subtly distinct from nauseated in this sense. Nauseated is a passive participle, and hence suggests a condition induced by a specific external cause. By contrast, nauseous is an adjective that refers to an occurrent state whose cause may be nonspecific or unknown. The person who reports that I woke up this morning feeling nauseous might not be willing to accept that he or she had been nauseated by any external agent.

To that, I say phah! Rest assured that I'll still look cross-eyed at you if you use nauseous in this sense, and further I will, with a concerned demeanor, press quite fully to discover the cause of your nausea. So don't think you can hide your wicked ways from me with semantics! :P

Footnote: in leafing through the dictionary, I discovered a word with fascinating spelling--phthisic. Look it up yourself (and all its phth brethren, like phthiriasis).