Microsoft.Office.Core

This namespace contains the core objects that are shared across all the different Office 365 applications.
This file is not a Primary Interop Assembly and is just a COM Type Library.
The latest assembly for Office 365 can be found in this folder

Folder - C:\WINDOWS\assembly\GAC_MSIL\Office\15.0.0.0__71e9bce111e9429c\Office.dll 

This can be added from the Reference Manager, COM, Type Libraries tab.
This can be accessed in code using the following namespace.

Office = Microsoft.Office.Core 

If you want to access the latest object model, this is the latest file and should be used instead of the 2013 "Office.dll" PIA file that Visual Studio uses by default.

public Microsoft.Office.Core.IRibbonUI _ribbonUI 

Previous Versions

Office 365 - Microsoft Office 16 Object Library
Office 2019 - Microsoft Office 16 Object Library
Office 2016 - Microsoft Office 16 Object Library
Office 2013 - Microsoft Office 15 Object Library
Office 2010 - Microsoft Office 14 Object Library
Office 2007 - Microsoft Office 12 Object Library