Back-end, also known as server-side development, refers to the part of web development that deals with the server-side logic, database management, and application programming interfaces (APIs) that power the front-end of a website or application.

Back-end developers use programming languages such as PHP, Python, Ruby, and Java to build server-side logic that powers the website or application. They work to ensure that the website or application is scalable, secure, and efficient, and that data is stored and managed in a way that can be accessed and used by the front-end.

Back-end developers may also work on the development of APIs, which are used to enable communication between the front-end and back-end of a website or application. APIs allow front-end developers to access data and functionality from the back-end, such as user authentication, database queries, and content management.

Some popular back-end development frameworks and libraries include Node.js, Django, Flask, Ruby on Rails, and Laravel.