In computer science, source code is text written in a computer programming language. 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 code, which can then be automatically translated to binary machine code that the computer can directly read and execute. An interpreter translates to machine code and executes it on the fly, while acompiler only translates to machine code that it stores as executable files; these can then be executed as a separate step.

Most computer applications are distributed in a form that includes executable files, but not their source code, which is useful to a user, programmer, orsystem administrator who wishes to modify the program or understand how it works.

The source code which constitutes a program is usually held in one or more text files stored on a computer's hard disk; usually these files are carefully arranged into a directory tree, known as a source tree. Source code can also be stored in a database (as is common for stored procedures) or elsewhere.

Source code also appears in books and other media; often in the form of small code snippets, but occasionally complete code bases; a well-known case is the source code of PGP.

The notion of source code may also be taken more broadly, to include machine code and notations in graphical languages, neither of which are textual in nature. An example from an article presented on the annual IEEE conference on Source Code Analysis and Manipulation:^[1]

For the purpose of clarity ‘source code’ is taken to mean any fully executable description of a software system. It is therefore so construed as to include machine code, very high level languages and executable graphical representations of systems.^[2]

The code base of a programming project is the larger collection of all the source code of all the computer programs which make up the project.

It has become common practice to maintain code bases in version control systems.