En. the 5th letter of the Roman alphabet ; en. the base of the natural system of logarithms ; approximately equal to 2.718282... ; En. the cardinal compass point that is at 90 degrees ; En. a radioactive transuranic element produced by bombarding plutonium with neutrons ; En. a fat-soluble vitamin that is essential for normal reproduction ; an important antioxidant that neutralizes free radicals in the body ; E layern. a region of the ionosphere (from 50 to 90 miles up) that reflects radio waves of medium length ; E regionn. a region of the ionosphere (from 50 to 90 miles up) that reflects radio waves of medium length ; e'enadv. even ; e'eradv. at all times ; all the time and on every occasion ; E-bombn. a bomb that explodes in midair and releases a massive burst of electromagnetic energy sufficient to disable computers and telecommunications without killing people or damaging buildings ; e-commercen. commerce conducted electronically (as on the internet) ; E-LASIKn. a refractive surgery procedure that reshapes the cornea ; e-mailn. (computer science) a system of world-wide electronic communication in which a computer user can compose a message at one terminal that can be regenerated at the recipient's terminal when the recipient logs in ; e-mailv. communicate electronically on the computer ; E. A. von Willebrandn. Finnish physician who first described vascular hemophilia (1870-1949) ;
