The Collaborative International Dictionary of English v.0.48:

G suit \G suit\ G-suit \G-suit\ n. [gravity + suit. 1944[MW10]]
   (Aeronautics)
   A suit worn by aviators and astronauts, designed to
   counteract the effects of high accelerations experienced in
   aerial maneuvers, in particular to avoid unconsciousness;
   called also anti-g suit. It exerts force on the abdomen and
   legs to prevent blackouts by counteracting the tendency of
   blood to accumulate below the heart, thus reducing the supply
   to the brain, when the forces due to acceleration are
   directed from head to toe.
   [PJC]

WordNet (r) 3.0 (2006):

anti-G suit
    n 1: worn by fliers and astronauts to counteract the forces of
         gravity and acceleration [syn: anti-G suit, G suit]