A technique for tracking a single request as it travels through many different services in a cloud application. Like following a package through the postal system — you can see every stop it made, how long it spent at each location, and where any delays occurred.
An online retailer uses AWS X-Ray to trace a customer's checkout request as it passes through 12 different microservices (cart, payment, inventory, shipping, notification), quickly identifying that a 3-second delay is caused by the inventory service making redundant database calls.
All four provide end-to-end request tracing across distributed services. AWS X-Ray and Azure Application Insights commonly integrate with OpenTelemetry; GCP Cloud Trace focuses on trace collection/analysis (often via OpenTelemetry); OCI APM provides tracing plus broader APM features like service maps and performance analytics.