If you’re nauseous, you feel queasy and sick to your stomach, and you might feel like vomiting. Could be the stomach flu or too much food before that roller-coaster ride.
Nauseous comes from the Latin word nausea, which means "seasickness." That's certainly one reason for getting nauseous, but we don't limit this word to the seafaring world anymore. Not only do we use this adjective when we're feeling queasy, but we also use it to describe whatever is making us feel queasy. A smell that turns your stomach is a nauseous smell. We also use nauseous figuratively when we're "grossed out" by someone who's overly romantic or self-involved.