Manifold Air Pressure Sensor

The MAP sensor (Manifold Air Pressure Sensor) is a main input to the ECM or engine computer. The MAP sensor measures the difference in air pressure between the intake manifold and atmospheric pressure. The ECM infers engine load from the MAP sensor reading.The MAP sensor converts engine vacuum and boost pressure into a varying output voltage, between 0-5 volts.

Manifold: The primary function of the intake manifold is to evenly distribute the combustion mixture (or just air in a direct injection engine) to each intake port in the cylinder head(s). Even distribution is important to optimize the efficiency and performance of the engine. (source: wikipedia)