ACE

Also: HDP, Hurricane Destruction Potential, Accumulated Cyclone Energy

Accumulated Cyclone Energy

"..is a measure used by the National Oceanic and Atmospheric Administration (NOAA) to express the activity of individual tropical cyclones and entire tropical cyclone seasons, particularly the North Atlantic hurricane season. It uses an approximation of the energy used by a tropical system over its lifetime and is calculated every six-hour period. The ACE of a season is the sum of the ACEs for each storm and takes into account the number, strength, and duration of all the tropical storms in the season."

"...A related quantity is hurricane destruction potential (HDP), which is ACE but only calculated for the time where the system is a hurricane"

Source here: See also...

or

Aerosol Characterization Experiment

A type of research program that gathers measurements from the atmosphere to characterize the properties and behavior of airborne particles called Aerosols. See for example here:

Definition based on (revised from) IPCC AR4.