Aggressive driving is defined by the National Highway Traffic Safety Administration as the behaviour of an individual who "commits a combination of moving traffic offences so as to endanger other persons or property."
In the UK, Road Drivers gives a common sense definition of aggressive driving:
Aggressive driving: The use of a motor vehicle in a deliberate and aggressive manner that is likely to endanger life by increasing the risk of a collision". This behavior is usually motivated by impatience, annoyance, hostility or an attempt to save time.— Road Drivers
There are other alternative definitions:
Aggressive driving behavior takes many forms. Typical aggressive driving behaviors include speeding, driving too close to the car in front, not respecting traffic regulations, improper lane changing or weaving, etc. The list is long. Most people drive aggressively from time to time and many drivers are not even aware when they are doing it.
Aggressive driving is difficult to define because of its many different manifestations but having a clear definition is important for police and legal action against it to succeed. A Global Web Conference on Aggressive Driving Issues organized in Canada in October 2000 offered the following definition “A driving behavior is aggressive if it is deliberate, likely to increase the risk of collision and is motivated by impatience, annoyance, hostility and/or an attempt to save time.— Aggressive driving behavior (background paper) Fourth Road Safety Week (5 - 11 April 2004)
By definition, aggressive driving is 'committing unprovoked attacks on other drivers', attacks such as not yielding to vehicles wishing to pass. The U.S. National Highway Traffic Safety Administration (NHTSA) has implemented the Fatality Analysis Reporting System,  which identifies actions that would fall under the category of aggressive driving, including:
According to the Fatality Analysis Reporting System, aggressive driving played a role in 56% of fatal crashes between 2003 and 2007, most of which were attributed to excessive speed. Aggressive driving also negatively impacts the environment as it burns 37% more fuel and produces more toxic fumes.