WordNet (r) 3.0 (2006):

life force
    n 1: (biology) a hypothetical force (not physical or chemical)
         once thought by Henri Bergson to cause the evolution and
         development of organisms [syn: life force, vital force,
         vitality, elan vital]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "life force":
   anima, animating force, atman, bathmism, beating heart,
   biological clock, biorhythm, blood, breath, breath of life,
   divine breath, divine spark, elan vital, essence of life,
   force of life, growth force, heart, heartbeat, heartblood,
   impulse of life, inspiriting force, jiva, jivatma, life breath,
   life cycle, life essence, life principle, life process, lifeblood,
   living force, pneuma, prana, seat of life, soul, spark of life,
   spirit, vis vitae, vis vitalis, vital energy, vital flame,
   vital fluid, vital force, vital principle, vital spark,
   vital spirit