Just would like to make clear, I cannot find straight answer. C# is general specification of this language done by MS, while Visual C# is implementation of this language, ag
For all the answers that "Visual C# is C# developed in Visual Studio" - I would point out that the C# Compiler (CSC.EXE) is not part of Visual Studio but is delivered as part of the Microsoft .net Framework - which is completely independent of the Microsoft Visual Studio line of development tools.
In that it has any meaning at all (beyond the initial marketing idea that it would appeal to Visual Basic developers, who were the majority of Windows developers on launch day) I'd say it refers to C# compiled by the Microsoft implementation of the .net Framework.
Whether or not Roslyn compiles "Visual C#" or "C#" is anyones guess.