This short post answers the question, "What key is the Host key in Oracle's VirtualBox on Windows, Mac and Linux."
Windows and Linux Host
The right Ctrl key is the Host key on a Windows and Linux host
The left Command key is the Host key on a Mac host
The Host key is also presented in the lower right corner of the window running the VM:
1.9.2. Capturing and Releasing Keyboard and Mouse in the Oracle® VM VirtualBox® User Manual at [link]