SEER stands for Seasonal Energy Efficiency Ratio. It indicates the kW of thermal energy generated by a unit for every kW of electricity consumed in cooling mode.
The higher the number the more efficient the equipment is in cooling mode. Unlike EER, it attempts to account for seasonality (or variances in ambient or outdoor temperatures). SEER is an energy performance indicator that represents the expected performance of an air conditioning unit in cooling mode over a range of outdoor (or ambient) weather conditions through the year. It indicates the kW of thermal energy generated by a unit for every kW of electricity consumed (depending on the country it could be represented by other units of measurement, such as BTU):
SEER = Output during a typical cooling-season / Input Electrical Energy
While the EER (Energy Efficiency Ratio), another term you will probably also see, simulates the operation of the unit at full load and a fixed outdoor and indoor temperature, SEER is calculated at partial loads and different indoor/outdoor temperatures for different proportions of time. Depending the country or region there will be slightly different standard for calculating SEER including a different blend of outdoor and indoor temperatures and different load factors.