An all-digital TV broadcast signal that delivers a high-resolution, wide-screen picture and 6 channels of digital sound. A resolution of 1,080 lines is considered high definition imagery, although 720 lines of progressive scanning has now come to be considered high resolution as well.
Source:
http://www.nielsenmedia.com/glossary/
http://www.arbitron.com/downloads/terms_brochure.pdf
