Window
Rectangular UI region owned by an application. Born at Xerox PARC ~1973 with Smalltalk-72. Macintosh (1984) made windows mainstream. Microsoft named its OS after the concept.
Window manager (X11, GNOME Mutter, KDE KWin) controls position, focus, decoration. Compositing (Wayland, Quartz, DWM) renders transparency + shadows + animations. Tiling window managers (i3, sway, dwm) place windows non-overlapping. Web browser windows are a different concept (BOM window object) — contain location, history, screen properties.