.NET Standard
A historical spec that let one library run on both .NET Framework and modern .NET.
.NET Standard is a versioned set of APIs that multiple .NET implementations promised to support; it is no longer evolved, and new libraries target modern .NET directly.
Examples
A library with
<TargetFramework>netstandard2.0</TargetFramework>can be used from both .NET Framework and modern .NET.
See also