The ---- is the term used to describe the window that is currently being used.
The Active Window is the term used to describe the window that is currently being used.
An active window is the currently focused window in the current window manager or explorer. Different window managers indicate the currently-active window in different ways and allow the user to switch between windows in different ways.
The active window may not always lie in front of all other windows on the screen. The active window is simply the window to which keys typed on the keyboard are sent; it may be visually obscured by other windows. This is especially true in window managers which do not require a click to change active windows.
In Windows, the active window is indicated by having a different coloured title bar. Clicking is not the only way of selecting an active window, however: some window managers (such as FVWM) make the window under the mouse pointer active—simply moving the mouse is sufficient to switch windows; a click is not needed.