Topmost is a universal term describing the object that is displayed on top of the others. It is usually used in connection to windows in Desktop Environments.
Topmost as a term is used to describe object with the highest z-index value. The object can be either a window (in Desktop Environment) or a HTML Element (on websites). Unlike modal, topmost allows window switching and manipulations. An example of modal window is Window's Task Manager, which stays on top by design.