What does 'Cor' stand for?

"Common Object Runtime"

For more, see:

• http://www.danielmoth.com/Blog/2005/05/mscorlibdll.html

cor: Before .NET was chosen as the name, this new platform was a successor to COM so it was codenamed COM 3.0 and then the name chosen was… Common Object Runtime (cor) and that is where mscorlib derives its name from (and that stuck regardless of the fact that .NET was the final name)!

EDIT: Here's an interesting addendum from Jeffrey Richter's book

• quoted from http://weblogs.asp.net/mreynolds/archive/2004/01/31/65551.aspx

When Microsoft first started working on the .NET Framework, MSCorLib.dll was an acronym for Microsoft Common Object Runtime Library. Once ECMA started to standardize the CLR and parts of the FCL, MSCorLib.dll officially became the acronym for Multilanguage Standard Common Object Runtime Library.

From the book: C# 6.0 in a Nutshell - The Definitive Reference (page 199)

Some of the .NET types are used directly by the CLR and are essential for the managed hosting environment. These types reside in an assembly called mscorlib.dll and include C#’s built-in types, as well as the basic collection classes, types for stream processing, serialization, reflection, threading, and native interoperability (“mscorlib” is an abbreviation for Multi-language Standard Common Object Runtime Library).

So it is supposed to be Multi-language Standard Common Object Runtime Library