KeyWords used by a system which may not be used as names of variables, functions or constants.

Most compilers, languages, and systems maintain a list of the words in a ReservedWordList, available to the user through help or other IDE features.

"Algol60 was the first programming language to have ReservedWords."

ALGOL 60 had, in fact _no_ reserved words. What would be keywords in today's languages were in fact separate symbols in the language, distinguished from identifiers by a StroppingConvention. The most common ones were the use of underlining or apostrophes:

   'if' x < 0 'then' x := -x;
or
   i_f_ x < 0 t_h_e_n_ x := -x;
However, not all ALGOL compilers implemented a StroppingConvention. Leaving symbols such as '''if''' and '''then''' unstropped meant, of course, that they had to be reserved. This was a violation of the standard, but common nonetheless.

In the reference language, these symbols were most commonly denoted in boldface, so in a publication the above examples would read:

   '''if''' ''x'' < 0 '''then''' ''x'' := - ''x'';
The upshot of all this is that one could write

   '''if''' ''if'' < 0 '''then''' ''if'' := - ''if'';
but I don't think it would have been encouraged.
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As opposed to RegisteredWords in Hyper ArtifactOrientedProgramming