Nausea is an uneasy sensation that you feel the need to vomit, however it does not always lead to vomiting. Nausea is not a condition in its own right as it is often a symptom of other conditions. Some common causes can include; viral infections which can be also known as a "stomach flu", eating or drinking too much, food poisoning, pregnancy and motion sickness or seasickness.
Nausea is often nothing to worry about in adults and will often clear within a day or two. How ever if it is persistent bouts of nausea that you are experiencing you should seek the help of your GP.