There are two aspects to the definition of a function: declaration and definition.
Declaration only tells enough about the function so that it may be called. The definition includes all of the declaration and all of the code. The declaration is included in the definition. The declaration alone is called a function prototype, or sometimes a function header. These prototypes are the main contents of header files, which are what is obtained by a #include statement.
For more on function prototypes, see the declare exhibit. Otherwise see the define exhibit.