The concept of the staging area is one which many newcomers struggle with when starting to learn git, and the fact that this concept is barely mentioned in the official documentation (i.e. man pages) doesn’t help at all. Instead the official name is “the index” which has absolutely nothing to do with the way users interact with it. That’s the reason most people who teach git prefer to use the term “staging area” instead, and in fact that’s the term used in the best available documentation: the Pro Git book (which is not official), and also pretty much in all online documentation, including tutorials and blog posts (e.g. Atlassian saving changes, code refinery).
Attempts to move away from the incorrect term “the index” towards one that most native and non-native English speakers can grasp without the need for further explanation have been attempted for more than a decade, and even though there’s universal consensus that “staging area” is by far the best alternative, attempts to use it officially in the documentation and user interface have been blocked because one person cannot be convinced.