The Collaborative International Dictionary of English v.0.48:

Liquid air \Liq"uid air\ (Physics)
   A transparent limpid liquid, slightly blue in color,
   consisting of a mixture of liquefied oxygen and nitrogen. It
   is prepared by subjecting air to great pressure and then
   cooling it by its own expansion to a temperature below the
   boiling point of its constituents (N: -194[deg] C; O:
   -183[deg] C.).
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

liquid air
    n 1: air in a liquid state