What's MSIL in .net framework?

Microsoft Intermediate Language, An intermediate language understands by the Just In Time (Compiler For .NET).
When compiling to managed code, the .NET Common Language Runtime compiler translates your source code into Microsoft intermediate language.
Furthermore the .NET Just In Time compiler will convert that code into computer native code.
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http://msdn.microsoft.com/en-us/library/c5tkafs1%28v=vs.71%29.aspx

The Microsoft Intermediate Language (MSIL) is an old name of the Common Intermediate Language (CIL) that was used in the bete releases.

(CLI) previously known as (MSIL) is the lowest-level human-readable programming languagede fined by the Common Language Infrastructure (CLI) specification and is used by the .NET Framework and Mono Languages which target a CLI-compatible runtime environment compile to CIL, which is assembled into an object code that has a bytecode-style format. CIL is an object-oriented assembly language, and is entirely stack-based. Its bytecode is translated into native code or — most commonly — executed by a virtual machine

MSIL : (Microsoft Intermediate language) is machine independent code generated by .NET framework after the compilation of program written in any language by you.

(IL)Intermediate Language is also known as MSIL (Microsoft Intermediate Language) or CIL(Common Intermediate Language). All .NET source code is compiled to IL. This IL is thenconverted to machine code at the point where the software is installed, or at run-time by a Just-In-Time (JIT) compiler.