What aspect of the aircraft does the Altimeter specifically measure?

The altimeter specifically measures aircraft altitude by sensing changes in atmospheric pressure. As an aircraft ascends or descends, the air pressure outside the aircraft changes, which the altimeter interprets to determine how high or low the aircraft is above sea level. This functionality is critical for maintaining safe flight operations, as pilots need to know their altitude to avoid obstacles, comply with air traffic control instructions, and ensure safe landings at airports.

In the context of the provided choices, other options focus on different aspects of flight and aircraft operations. For instance, measuring weight is unrelated to altitude determination, and the pressure in the cabin is pertinent to the internal environment rather than the aircraft's position relative to sea level. Similarly, velocity over ground is concerned with the speed at which the aircraft is moving relative to the ground rather than how high it is flying. This highlights that while altitude is key for navigation and safety, each measurement serves a distinct purpose in understanding aircraft performance and environmental conditions.