The pump at the heart of an AC or heat pump that pressurizes refrigerant to move heat. It's the most expensive single component — which is why a failed compressor on an older system usually triggers the repair-vs-replace decision.
Around Baltimore, Annapolis, and the DC metro, cooling decisions revolve around humid Mid-Atlantic summers, correct sizing for row houses and older homes, SEER2 efficiency, and whether a repair or a planned replacement is the smarter long-term spend.