Plain English Meaning
A check, done by a qualified shop, that proves the altimeter is reading altitude correctly and that the system feeding it pressure is sealed and accurate. It must be current within the last 24 calendar months for IFR flight.
Full Definition
A required inspection and check of an aircraft's altimeter system and static pressure system to confirm it meets the accuracy and integrity standards specified in 14 CFR Part 91.411. For IFR flight in controlled airspace, this test must have been performed and recorded within the preceding 24 calendar months.