What is the definition of Pressure?

Pressure can be defined as the force per unit area. 

p = F/A

The units are therefore N/m².

1 N/m² is 1 Pa. 

The Pascal is the standard unit of pressure.

The force is the mass x acceleration (of the gas molecule), F = ma

The acceleration of the gas molecule is the rate of change of velocity, v/t

The momentum of a gas molecule is a product of the mass and its velocity, mv.