Original: It is defined as the pressure exerted by a column of mercury of 1 inch in height at 32 Â F (0 Â C) at the standard acceleration of gravity.
Simplified: It is the amount of pressure that a column of mercury that is one inch tall has at 32 Â F (0 Â C) at the standard acceleration of gravity, or how fast things speed up as they are falling because of gravity.