The 1974 World Food Summit defined food security as:
... availability at all times of adequate world food supplies of basic foodstuffs to sustain a steady expansion of food consumption and to offset fluctuations in production and prices. (Report of the World Food Conference, Rome 5-16 November 1974. New York)
In chapter 2 of the FAO publication, Trade Reforms and Food Security: conceptualizing the linkages, the definition of the term Food Security is presented as a flexible concept which has evolved over time.
The Committee on World Food Security in document CFS 2012/39/4 has provided further official definition to Food Security and related terms.
For more information on how the definition has evolved, see the FAO publication: Trade Reforms and Food Security: Conceptualizing the Linkages- Chapter 2:2