a simple and universal interface between web servers and web applications or frameworks: the Python Web Server Gateway Interface (WSGI).

WSGI is the Web Server Gateway Interface. It is a specification that describes how a web server communicates with web applications, and how web applications can be chained together to process one request.

WSGI is a Python standard described in detail in PEP 3333.

https://www.python.org/dev/peps/pep-3333/

Original Rationale and Goals (from PEP 333)

Python currently boasts a wide variety of web application frameworks, such as Zope, Quixote, Webware, SkunkWeb, PSO, and Twisted Web -- to name just a few [1]. This wide variety of choices can be a problem for new Python users, because generally speaking, their choice of web framework will limit their choice of usable web servers, and vice versa.

【类似 Java servelet api  服务器和web开发框架之间的接口规范  】

By contrast, although Java has just as many web application frameworks available, Java's "servlet" API makes it possible for applications written with any Java web application framework to run in any web server that supports the servlet API.

The availability and widespread use of such an API in web servers for Python -- whether those servers are written in Python (e.g. Medusa), embed Python (e.g. mod_python), or invoke Python via a gateway protocol (e.g. CGI, FastCGI, etc.) -- would separate choice of framework from choice of web server, freeing users to choose a pairing that suits them, while freeing framework and server developers to focus on their preferred area of specialization.

This PEP, therefore, proposes a simple and universal interface between web servers and web applications or frameworks: the Python Web Server Gateway Interface (WSGI).

But the mere existence of a WSGI spec does nothing to address the existing state of servers and frameworks for Python web applications. Server and framework authors and maintainers must actually implement WSGI for there to be any effect.

WSGI, Web Server Gateway Interface
如全称代表的那样，WSGI不是服务器，不是API，不是Python模块，更不是什么框架，而是一种服务器和客户端交互的接口规范！
更具体的规范说明请搜索“PEP 3333”。
在WSGI规范下，web组件被分成三类：client, server, and middleware.
WSGI apps(服从该规范的应用)能够被连接起来(be stacked)处理一个request，这也就引发了中间件这个概念，中间件同时实现c端和s端的接口，c看它是上游s，s看它是下游的c。
WSGI的s端所做的工作仅仅是接收请求，传给application（做处理），然后将结果response给middleware或client.除此以外的工作都交给中间件或者application来做。

https://www.zhihu.com/question/19998865/answer/26203965