The definition of a 30 SEER AC is an air conditioning system that has a Seasonal Energy Efficiency Ratio (SEER) of 30 or higher. SEER is a measure of the efficiency of an air conditioner, and the higher the SEER rating, the more efficient the system is. A 30 SEER AC is considered to be one of the most efficient air conditioning systems available, as it is able to cool a space more efficiently than other systems with lower SEER ratings. This type of system is ideal for those who want to save money on their energy bills, as it uses less energy to cool a space than other systems. Additionally, a 30 SEER AC is quieter than other systems, making it a great choice for those who want to reduce noise levels in their home. The system also has a longer lifespan than other systems, making it a great investment for those who want to save money in the long run.