What do you mean by translation unit?

A Translation Unit is a set of source files that is seen by the complier and it translate it as one unit which is generally ?.c? file and all the header files mentioned in ‪#‎include‬ directives.
When a C preprocessor expands the source file with all the header files the result is the preprocessing translation unit which when futher processed translates the preprocessing translation unit into translation unit, furtherwith the help of this translation unit compiler forms the object file and ultimately forms an executable program.