MAP
The engine has a fixed displacement. Therefore it is constantly trying to draft a certain amount of air through the intake. The airflow is being restricted by some means (Butterfly, slidevalve,etc). Due to the engine attempting to draft more air than the butterfly is allowing to pass through, there is a "partial vacuum", or a pressure lower than atmospheric at the intake port. As the butterfly opens more and more, the restriction is lessened allowing the MAP to closer approach atmospheric pressure