In the automotive sense of the term, a blind spot is the area outside the normal field of vision that can't be seen without turning around or turning the head. Blind spots generally occur due to the inability of side and rear view mirrors to show everything to the sides and behind a vehicle. Larger vehicles, such as semi-trucks and motor homes, typically have larger blind spots, but even cars, trucks, SUVs and boats usually have a blind spot or two.
To be as safe as possible out on the road, it's important to determine where the blind spots are in any particular vehicle before driving it. The easiest way to do this is to sit in the driver's seat, look in the mirrors, and then turn around and compare the two views. Anything that's visible by turning around but doesn't appear in the mirrors is considered to be in a blind spot.
The blind spot can become especially dangerous when doing things such as changing lanes or parallel parking. If there is a blind spot on either side of a vehicle, it becomes possible to accidentally merge into a space already occupied by another vehicle, with costly and sometimes dangerous consequences. If the blind spot is known, a quick look over the shoulder can alleviate all possible chance of an accident. Similarly, blind spots can make it difficult or impossible to parallel park without hitting the other vehicles. Turning around to assess the situation can fill in the missing information from the blind spots, allowing for a safe parking job.
It is also important to consider the blind spots of other drivers when out on the road. Many people don't think to look over their shoulders when turning or changing lanes, so when driving in another driver's blind spot, it is always safest to assume they don't know anyone is there. This is especially important when considering larger vehicles, as their blind spots might cause an entire vehicle to virtually disappear while alongside it. It is generally safe to assume that if the mirrors on a large vehicle can't be seen, that driver doesn't know another vehicle is present. If their mirrors are adjusted incorrectly, however, they might not be able to see even if their mirrors are visible to another driver.
Driving large vehicles also presents several unique problems. With semi-trucks and some larger motor homes, it's simply impossible to see everything in a blind spot by a simple turn of the head. Large mirrors can sometimes minimize blind spots, and rear and side view cameras can allow drivers of very long vehicles to see into their blind spots. Measures such as these can make the roads much safer for everyone by virtually eliminating blind spots altogether.