.NET 5 is what should have been .NET Core 4.x. Microsoft instead renamed it for two reason according to an article they posted.
"We skipped version numbers 4.x to avoid confusion with .NET Framework 4.x.
We dropped "Core" from the name to emphasize that this is the main implementation of .NET going forward. .NET 5.0 supports more types of apps and more platforms than .NET Core or .NET Framework.
ASP.NET Core 5.0 is based on .NET 5.0 but retains the name "Core" to avoid confusing it with ASP.NET MVC 5. Likewise, Entity Framework Core 5.0 retains the name "Core" to avoid confusing it with Entity Framework 5 and 6."