Condition that exists when less air is supplied to a space than is exhausted from the space, so the air pressure within that space is less than that in surrounding areas. Under this condition, if an opening exists, air will flow from surrounding areas into the negatively pressurized space.
Condition in a building where the inside pressure is less than the outside pressure. This is often caused by kitchen fans that draw inside air out of the house, and it can effect stove or fireplace performance.
Pressure is defined in terms of a force applied over an area. In many physics problems we consider idealised situations; typically a single, positive ("pushing") force acting on a surface. For example, an ideal gas is modelled as particles (eg. air molecules) that strike (and elastically bounce away from) the walls of its container.