Plain English Meaning
A test done at least every two years to make sure the aircraft's altitude-measuring equipment is leak-free and reading correctly, including the part that automatically sends the altitude to controllers.
Full Definition
A required check of the static pressure system, altimeter, and automatic altitude reporting equipment, performed by an appropriately certified person, that must be completed within the preceding 24 calendar months for the aircraft to be operated under instrument flight rules in controlled airspace. The inspection verifies the system holds pressure correctly, the altimeter reads accurately within tolerances, and the encoding altimeter reports the correct altitude to air traffic control.