Source code is often defined to be the files that contain instructions written in computer programming languages like Java, C++, Swift, etc. However, I have found that most every commercial software project comprises other types of files as well.

Therefore, an alternate definition of source code could include all the files authored by humans which express the functioning and logic to be implemented by a computer.

With this alternate definition, computer source code would include instructions written in programming languages plus the other files used by those instructions to express the functioning and logic to be implemented by a computer. This often includes configuration files, interface files, markdown files, and data files. 

Indeed, configuration, interfaces, markdown, and data can be represented in instructions written in programming languages. However, representing configuration, interfaces, markdown, and data in their own files that are not written in a programming language provides modularity and clarity of purpose.

The fact that configuration, interfaces, markdown, and data are sometimes not represented in instructions written in programming languages does not make them less important to the functioning and logic to be implemented by the computer. As such, this alternate defintion of source code includes instructions written in programming languages as well as configuration files, interface files, markdown files, and data files.