Pilot licensing or certification refers to permits to fly aircraft that are issued by the Civil Aviation Authority (CAA) in each country, establishing that the holder has met a specific set of knowledge and experience requirements. This includes taking a flying test. The certified pilot can then exercise a specific set of privileges in that nation's airspace. Despite attempts to harmonize the requirements between nations, the differences in certification practices and standards from place to place serve to limit full international validity of the national qualifications. In addition, U.S. pilots are certified, not licensed, although the word license is still commonly used informally. Legally, pilot certificates can be revoked by administrative action, whereas licensing (e.g., a driver's license) requires intervention by the judiciary system.