I think at last there is not only one correct answer for this, because there are several possibilitys to mention what an LaTeX input file does.
Similar to programming languages like FORTRAN, Algol68, Pascal, C, C++ the written code for this programming languages is usually called "source code", because this code is compiled and changed to an executable file.
Because you can see the translating of the LaTeX input file to an printable pdf document the same process done with programming languages (and programming languages exists first) the well known words "source code" is often used.
But there exist some other word combinations expressing at last the same:
See for example:
lshort.pdf (page 8, point 1): LaTeX input file
usrguide.pdf: Source Code
- tex.stackexchange.com: LaTeX source file or source file for LaTeX
- Wikibook: See chapter: 2.3. Editors: TeX and LaTeX source documents
- http://tug.org/levels.html:: See Output formats: TeX source files* ...
To summarize: TeX/LaTeX source file/code/document or TeX/LaTeX input file means at last the same. Sometimes with TeX/LaTeX, sometimes without ...
Open lshort.pdf and usrguide.pdf with
texdoc lshort and