Additional Dialogue Recording. This is just Dubbing, done in addition to or as a substitution for Location Sound. The term ADR, being something of a mysterious acronym, has a certain appeal, as it obscures the fact that dubbing was involved when it appears in the credits of your film. This might have something to do with the current prevalence of the term.
Glossary of Film Terms - http://homepage.newschool.edu/~schlemoj/film_courses/glossary_of_film_terms/
reprinted with permission