Originally Posted by Maroof Hussain Chishty

Airworthiness is a term used to describe whether an aircraft has been certified as suitable for safe flight. Certification is initially conferred by a Certificate of Airworthiness (CofA) from a National Airworthiness Authority, and is maintained by performing required maintenance actions by a licensed Aircraft Maintenance Engineer. Required scheduled maintenance actions are detailed in the Aircraft's System of Maintenance. Minor inspections are typically every 100 flight hours, with more intensive maintenance inspections at longer intervals.

The application of airworthiness defines the condition of an aircraft and supplies the basis for judgment of the suitability for flight of that aircraft, in that it has been designed with engineering rigor, constructed, maintained and is expected to be operated to approved standards and limitations, by competent and approved individuals, who are acting as members of an approved organization and whose work is both certified as correct and accepted on behalf of the State.

One notable example of an aircraft that was not legally airworthy is Larry Walters' "Lawn chair flight." on July 2, 1982 in a homemade "aircraft" he called Inspiration I.

In the U.S., Title 14, Code of Federal Regulations, Subchapter F, Part 91.7 states: "a) No person may operate an aircraft unless it is in an airworthy condition. b) The pilot in command of a civil aircraft is responsible for determining whether that aircraft is in condition for safe flight. The pilot in command shall discontinue the flight when unairworthy mechanical, electrical, or structural conditions occur."

source
http://en.wikipedia.org/wiki/Airworthiness