WordNet (r) 3.0 (2006):

lifework
    n 1: the principal work of your career

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "lifework":
   activity, art, business, calling, campaign, career,
   career building, careerism, cause, commitment, craft, crusade,
   drive, faith, game, great cause, handicraft, interest, issue, line,
   line of business, line of work, mass movement, metier, mission,
   movement, mystery, number, occupation, practice, principle,
   profession, pursuit, racket, reason for being, specialization,
   specialty, trade, vocation, walk, walk of life, work