变结构系统简称VSS，是一种不连续的非线性系统，形式如下：

其中为状态矢量，为时间变量， 为分段连续函数。因为系统分段连续的特性，其系统行为会随其状态而改变，而且会像是数个不同的连续非线性系统。在其边界处，而特性会有剧烈的变化。因此其结构会随其状态而不同。

以下就是一个变结构系统的例子

变结构控制的发展也和变结构系统的分析方式有关，都是混合系统中的特例。

A variable structure system, or VSS, is a discontinuous nonlinear system of the form

where is the state vector, is the time variable, and is a piecewise continuous function. Due to the piecewise continuity of these systems, they behave like different continuous nonlinear systems in different regions of their state space. At the boundaries of these regions, their dynamics switch abruptly. Hence, their structure varies over different parts of their state space.