SEER, or Seasonal Energy Efficiency Ratio, is a rating used to measure the energy efficiency of an air conditioner or heat pump. It represents the cooling output in BTUs (British Thermal Units) during a typical cooling season divided by the total electrical energy input in watt-hours. A higher SEER rating indicates greater energy efficiency, meaning the air conditioner can provide the same cooling output while using less electricity. SEER ratings typically range from 13 to 25 or higher for modern systems, with higher SEER-rated units being more energy-efficient but often coming at a higher upfront cost. Investing in a higher SEER-rated air conditioner can lead to lower energy bills and reduced environmental impact over the long term, especially in regions with prolonged cooling seasons.