What is "M" language?

Answer:

"M" is a new modeling language being developed by Microsoft to build domain models and working with Data. "M" standas for Model.

Another definition written by one of the writer (Aaron Skonnard) is

"A new text-based data modeling language, designed for developers, for describing the data stored in the repository and for generating SQL. The repository will come with numerous pre-defined schemas that represent common distributed application constructs. “M” also provides a grammar for creating custom Domain Specific Languages (DSL)."

For more details, visit http://msdn.microsoft.com/en-us/library/dd129539(VS.85).aspx