Source : Webster's Revised Unabridged Dictionary (1913)

Lifeblood \Life"blood`\ (l[imac]f"bl[u^]d`), n.
   1. The blood necessary to life; vital blood. --Dryden.

   2. Fig.: That which gives strength and energy.

            Money [is] the lifeblood of the nation. --Swift.

Source : WordNet®

lifeblood
     n 1: the blood considered as the seat of vitality
     2: an essential or life-giving force; "water is the lifeblood
        of India"