What is 'SEER'?

Seasonal Energy Efficiency Ratio (SEER) - A measure of seasonal or annual efficiency of a central air conditioner or air conditioning heat pump. It takes into account the variations in temperature that can occur within a season and is the average number of Btu of cooling delivered for every watt-hour of electricity used by the heat pump over a cooling season.

Think of it as "Miles Per Gallon" for an air conditioner. A bigger number is better. the energy codes for all USA air conditioning is a minimum of 13 SEER.

[from Energy Savers: Glossary at at The U.S. Department of Energy Website]