Update README.md (#1227)

README.md

@@ -16,7 +16,7 @@ microservices, cloud native and container-based (Docker, Kubernetes, Mesos) arch
 [![OpenTracing-1.x Badge](https://img.shields.io/badge/OpenTracing--1.x-enabled-blue.svg)](http://opentracing.io)
 
 # Abstract
-**SkyWalking** started as a distributed system in 2015. From 5.x, it evolved to the fully functional [Application Performance Management](https://en.wikipedia.org/wiki/Application_performance_management) 
+**SkyWalking** started as a distributed tracing system in 2015. From 5.x, it evolved to the fully functional [Application Performance Management](https://en.wikipedia.org/wiki/Application_performance_management) 
 system. It is used for tracing, monitoring, diagnose distributed systems, especially based on microservices, cloud native and container,
 including:
 - Distributed tracing and context propagation