窗口系统（windowing system）是以使用窗口作为主要特征之一的图形用户接口的构成组件。更为明确地说，是桌面环境的构成组件。窗口系统支撑着窗口管理员的实作（implementation）；对于「图像硬件（graphics hardware）、指向设备（pointing devices）像是鼠标与键盘等设备」提供基本支持。绘制鼠标光标，一般也与窗口系统相关。

从程序员的观点来看，窗口系统实作出绘图的基本单位，像是字体绘制，或是在屏幕上划线，有效地提供绘图硬件的一种抽象概念。

窗口系统能让电脑用户同时与数个程序一起工作。每个程序在他自己的窗口中运行，也就是屏幕上的矩形之中。大部份的窗口系统允许窗口重叠，且提供用户运行标准的操作，像是移动/改变窗口尺寸，把窗口送到前景/背景，和最小化/最大化一个窗口。

In computing, a windowing system (or window system) is a type of graphical user interface (GUI) which implements the WIMP (windows, icons, menus, pointer) paradigm for a user interface. Each currently running application is assigned a usually resizeable and usually rectangular shaped surface of the display to present its graphical user interface to the user; these windows may overlap each other, as opposed to a tiling interface where they are not allowed to overlap. Usually a window decoration is drawn around each window. The programming of both the window decoration and of available widgets inside of the window, which are graphical elements for direct user interaction, such as sliders, buttons, etc., is eased and simplified through the use of widget toolkits.