The Collaborative International Dictionary of English v.0.48:

main file \main file\ n. (computers)
   A file that is used as the authority in a given job and that
   is relatively permanent.

   Syn: master file.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

main file
    n 1: (computer science) a computer file that is used as the
         authority in a given job and that is relatively permanent
         [syn: master file, main file]