Plain English Meaning
The pressure of the still air around the airplane — the squeeze the atmosphere puts on everything, without any added push from the airplane moving through it.
Full Definition
The ambient atmospheric pressure of the air surrounding the aircraft, measured perpendicular to the airflow so that the motion of the aircraft through the air does not add to the reading. Static pressure is the pressure the air exerts simply by being there, independent of any movement.
Context Anchor
Seen in discussions of airflow, ground effect, and the instruments that use outside air pressure, such as the altimeter and vertical speed indicator.
Derivation
From Latin 'staticus' meaning 'standing still' or 'at rest.' Static pressure is the pressure of air considered as if it were at rest — the part of the total air pressure that is not caused by motion.