Eye line is the direction an actor should look off-screen to match a reverse angle or a P.O.V. shot. It is best to give the actor an actual thing or spot to look at rather than a blank spot on an empty wall or an empty space in mid air.
Glossary of Film Terms - http://homepage.newschool.edu/~schlemoj/film_courses/glossary_of_film_terms/
reprinted with permission