Definitions

Altimetry System Error The difference between the altitude indicated by the altimeter display, assuming a correct altimeter barometric setting, and the pressure altitude corresponding to the undisturbed ambient pressure. (ICAO doc 9574)

Put more simply, ASE is the difference between the altitude that the pilot, ground controller and aircraft systems believe the aircraft to be at and the actual altitude. To be compliant with international standards, the ASE of an aircraft must be less than 245 ft.

Aberrant Aircraft Those aircraft which exhibit measured height-keeping performance that is significantly different from the core height-keeping performance measured for the whole population of aircraft operating in RVSM airspace. (ICAO doc 9574)