"Originally, CoreCLR was the runtime of Silverlight and was designed to run on multiple platforms, specifically Windows and OS X. CoreCLR is now part of .NET Core and represents a simplified version of the CLR. It's still a cross-platform runtime, now including support for many Linux distributions. CoreCLR is also a virtual machine with JIT and code execution capabilities."
From https://docs.microsoft.com/enus/dotnet/standard/glossary :
"Originally, CoreCLR was the runtime of Silverlight and was designed to run on multiple platforms, specifically Windows and OS X. CoreCLR is now part of .NET Core and represents a simplified version of the CLR. It's still a cross-platform runtime, now including support for many Linux distributions. CoreCLR is also a virtual machine with JIT and code execution capabilities."