The evening television hours that precede and follow prime time, usually 4 p.m. to 7:30 p.m. (early fringe) and 11 p.m. to 1 a.m. (late fringe), all Eastern Standard Time. The prime access time 7:30 p.m. to 8:00 p.m. may be included.

Source:
http://www.nielsenmedia.com/glossary/
http://www.arbitron.com/downloads/terms_brochure.pdf