the blood, considered as essential to maintain life: to spill one's lifeblood in war.
a life-giving, vital, or animating element: Agriculture is the lifeblood of the country.

1580–90; life + blood Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
lifeblood (ˈlaɪfˌblʌd)
1.  the blood, considered as vital to sustain life
2.  the essential or animating force

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1580s, "blood necessary for life," from life + blood. Figurative and transferred use is from 1590s.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Few resources have been the focus of more attention than oil, the lifeblood of
  the modern world.
Those that debauch the currency dilute the lifeblood of the economy until it
  dies of suffocation.
More and more of the nation is having its economic lifeblood drained.
Volunteers are the lifeblood of a presidential campaign.
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