A G-suit, or the more accurately named anti-G suit, is worn by aviator

Aviator

An aviator is a person who flies an aircraft. The first recorded use of the term was in 1887, as a variation of 'aviation', from the Latin avis , coined in 1863 by G. de la Landelle in Aviation Ou Navigation Aérienne...

s and astronaut

Astronaut

An astronaut or cosmonaut is a person trained by a human spaceflight program to command, pilot, or serve as a crew member of a spacecraft....

s who are subject to high levels of acceleration

Acceleration

In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...

 force ('g

G-force

The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...

s). It is designed to prevent a black-out and G-LOC

G-LOC

G-LOC, pronounced 'GEE-lock', is the abbreviation of G-force induced Loss Of Consciousness, a term generally used in aerospace physiology to describe a loss of consciousness occurring from excessive and sustained g-forces draining blood away from the brain causing cerebral hypoxia...

 (g

G-force

The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...

-induced Loss Of Consciousness) caused by the blood pooling in the lower part of the body when under acceleration, thus depriving the brain of blood. Black-out and G-LOC has caused a number of fatal aircraft accidents.
If blood is allowed to pool in the lower areas of the body, the brain will be deprived of blood, leading to temporary hypoxia

Hypoxia (medical)

Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...

.