牛津词典第八版，是一本包含大量单词和解释的英语参考书。在其中，CSS是一个重要的条目。下面是牛津词典第八版中给出的CSS定义：

CSS (noun):Cascading Style Sheets; a system for controlling the presentation of HTML or XML documents by defining the formatting, layout, and appearance of their elements (such as fonts, colours, and margins) in one or more separate stylesheets (which can be included within the document or imported from external files). The term 'cascading' refers to the rules governing the application of styles to elements, which can be inherited from ancestors, overridden by local definitions, or merged from multiple sources in specific ways.

简单来说，CSS负责控制网页中各个元素的样式，例如字体、颜色、边距等。同时，CSS的样式也可以被继承、覆盖或合并。

作为web前端开发必备的技能之一，CSS已成为无数网站的基石。在牛津词典第八版中，它也得到了充分的体现。