Nausea is a general feeling of unease and discomfort in the stomach, often with the urge to vomit. The word nausea comes from the Latin word for seasickness.
Nausea is a symptom, rather than an illness or disease. The causes for it very often are not in the stomach itself, but somewhere else in the body. Nausea is often caused by a stomach virus. Nausea is usually harmless, in the short term. A good treatment for it can be not to take solid food (and to only drink non-alcoholic drinks, like water). Nausea can also occur during pregnancy, and is quite normal in that context. People can suffer nausea without vomiting.