Technically, the words nauseous and nauseating describe something that makes you feel sick, and the words nauseated and queasy describe the way you feel - like your stomach is uncomfortable and you might vomit:
A nauseous smell of rotten fruit came from the garbage bags.
That scene in the movie is nauseating; I wish they didn't show such graphic violence.
I felt nauseated every morning during the first few months of pregnancy.
My son was a little queasy during the boat ride.
However, today many people say "I feel nauseous" - so this use of nauseous to mean the feeling is becoming more acceptable. Nauseating is still used to describe the thing causing the feeling.
Nausea is the noun form (the other words are adjectives). It is often used when describing symptoms:
Possible side effects of this medication include fatigue, nausea, and headaches.
He was hospitalized with severe nausea and an elevated body temperature.
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