Computer software is said to have Internal Documentation if the notes on how and why various parts of code operate is included within the source code

Source code

In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...

 as comments. It is often combined with meaningful variable

Variable (programming)

In computer programming, a variable is a symbolic name given to some known or unknown quantity or information, for the purpose of allowing the name to be used independently of the information it represents...

 names with the intention of providing potential future programmers a means of understanding the workings of the code.

This contrasts with external documentation

Documentation

Documentation is a term used in several different ways. Generally, documentation refers to the process of providing evidence.Modules of Documentation are Helpful...

, where programmers keep their notes and explanations in a separate document.

Internal documentation has become increasingly popular as it cannot be lost, and any programmer working on the code is immediately made aware of its existence and has it readily available.

see also Literate programming

Literate programming

Literate programming is an approach to programming introduced by Donald Knuth as an alternative to the structured programming paradigm of the 1970s....