One often hears the term being applied to the limits that the human body can withstand without losing conciousness, sometimes referred to as "blacking out", or g-loc (loc stands for loss of consciousness). A typical person can handle about 5 g (50 m/s²) before this occurs, but through the combination of special g-suits and efforts to strain muscles —both of which act to force blood back into the brain— modern pilots can typically handle 9 g (90 m/s²) or more.
Resistance to "negative" or upward gees which drive blood to the head, is much less. This limit is typically in the -2 to -3 g (-20 to -30 m/s²) range. The vision goes red and is also referred to as a "red-out". This is probably due to capillaries in the eyes bursting under the increased blood pressure. Humans can survive brief intervals of about 20 to 40 g, and any exposure to around 100 g or more is lethal. -Wikipedia