The "Common Language Runtime" is also known as the "CLR". It defines an execution environment for programs. The best known implementations are DotNet and NovellMono. Because there are at least two viable implementations usable on real-world projects, the term "CLR" or "Common Language Runtime" is often used in place of writing ".NET".

External references:

 * http://en.wikipedia.org/wiki/Common_Language_Runtime
 * http://msdn.microsoft.com/en-us/library/cc265151(VS.95).aspx
 * http://msdn.microsoft.com/en-us/library/8bs2ecf4(VS.71).aspx