The Application object is the root object of the AutoCAD .NET API. 

From the Application object, you can access the main application window as well as any drawing open in memory. Once a drawing has been referenced, access the objects in the drawing can be accessed.

Autodesk.AutoCAD.ApplicationServices.Application

Public abstract Class Application
Inherits Autodesk.AutoCAD.ApplicationServices.Core.Application

public abstract class Application : Autodesk.AutoCAD.ApplicationServices.Core.Application;