An aircraft's static pressure system is the key input to its altimeter and, along with the pitot pressure system, also drives the airspeed indicator.
The static pressure system is open to the aircrafts exterior through a small opening called the static port, which allows sensing the ambient atmospheric pressure at the altitude at which the aircraft is flying. In flight, the air pressure varies slightly at different positions around the aircrafts exterior, so designers must select the static ports locations carefully. Wherever they are located, the air pressure that the ports observe will generally be affected by the aircrafts instantaneous angle of attack. The difference between that observed pressure and the actual atmospheric pressure (at altitude) causes a small position error in the instruments indicated altitude and airspeed. A designers objective in locating the static port is to minimize the resulting position error across the aircraft's operating range of weight and airspeed.
Many authors describe the atmospheric pressure at the altitude at which the aircraft is flying as the freestream static pressure. At least one author takes a different approach in order to avoid a need for the expression freestream static pressure. Gracey has written "The static pressure is the atmospheric pressure at the flight level of the aircraft". Gracey then refers to the air pressure at any point close to the aircraft as the local static pressure.