C and C++ lingo

Declaration - you’re telling the compiler “there’s something with this name, and it has this type”. Example:

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int hello();

Definition - providing all the details. For functions it means providing function body. Example:

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int hello()
{
  return 420;
}

For more info see:
What’s the difference between declaring and defining in C and C++

Take a .c file, put in a preprocessor - you get a translation unit that is turned into an .o file after compilation. See also on Wikipedia.