Revenue Passenger Kilometers (RPK) is a key metric used in the airline industry to measure passenger traffic. It represents the number of kilometers flown by paying passengers. For example, if 100 passengers travel 1,000 kilometers, the RPK would be 100,000 kilometers. RPK is important for airlines as it helps assess their performance and profitability. By analyzing RPK, airlines can understand demand trends, optimize routes, and make informed decisions about capacity and pricing. This metric is often used alongside other indicators like Available Seat Kilometers (ASK) to evaluate operational efficiency.