If you get carsick, you feel nauseated when you're in a moving vehicle. Kids who get carsick often feel worse when they try to read in a car.
If you get sick to your stomach whenever you ride in a car, you can say you tend to get carsick. This is just one of many kinds of motion sickness — some people feel ill when they're traveling by airplane, on a train, or on a boat. The word seasick came first, in the 16th century, and carsick was modeled after it, appearing around 1908.