One of the Google Searches that came by my site was "c# define".
Now I presume that the user was looking to see what you can do with "defines" in C#. Well if you compare them to C #defines then not much at all because in C you define macros that are substituted by the pre-processor to expand the code at compile time. In C#, defines are used to indicate what code to compile when the relevand command is passed in via the "Conditional Compilation symbols".
I don't think C# 2.0 changes this, I am not too sure about C# 3.0 though.