Passenger-kilometers is a measurement used in transportation to quantify the distance traveled by passengers. It is calculated by multiplying the number of passengers by the distance they travel. For example, if 10 passengers travel 100 kilometers, that results in 1,000 passenger-kilometers. This metric is commonly used in various modes of transport, including trains, buses, and airplanes. It helps transportation companies and policymakers assess the efficiency and usage of their services, enabling better planning and resource allocation to meet passenger demand.