Film term:
This is the emulsion position of the film. There are two possibilities, just as there are two sides to a piece of film. Camera original is B-Wind. A print struck from it will be A-Wind. This is because film is printed emulsion against emulsion.
To tell if a piece of film is A-Wind or B-Wind hold it up with the emulsion facing you. If it is A-Wind the image will read correctly, if it is B-Wind it will be a mirror image. A-Wind and B-Wind material usually cannot be mixed, unless you don’t mind things being mirror image or some material being soft in focus as a result of being printed base-to-emulsion (the solution is usually optical printing). But mostly, issues of A-Wind and B-Wind do not come up all that frequently. It usually only comes up when you have just completed your sound mix and the mixing house asks if you need a B-Wind track. If it is to be used with the negative from your camera the answer is “yes.”
Glossary of Film Terms - http://homepage.newschool.edu/~schlemoj/film_courses/glossary_of_film_terms/
reprinted with permission