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The Free On-line Dictionary of Computing (foldoc)

/g*-dahn'kn/ Ungrounded; impractical; not well-thought-out;
untried; untested.

"Gedanken" is a German word for "thought". A thought
experiment is one you carry out in your head. In physics, the
term "gedanken experiment" is used to refer to an experiment
that is impractical to carry out, but useful to consider
because it can be reasoned about theoretically. (A classic
gedanken experiment of relativity theory involves thinking
about a man in an elevator accelerating through space.)
Gedanken experiments are very useful in physics, but must be
used with care. It's too easy to idealise away some important
aspect of the real world in constructing the "apparatus".

Among hackers, accordingly, the word has a pejorative
connotation. It is typically used of a project, especially
one in artificial intelligence research, that is written up in
grand detail (typically as a Ph.D. thesis) without ever being
implemented to any great extent. Such a project is usually
perpetrated by people who aren't very good hackers or find
programming distasteful or are just in a hurry. A "gedanken
thesis" is usually marked by an obvious lack of intuition
about what is programmable and what is not, and about what
does and does not constitute a clear specification of an
algorithm. See also {AI-complete}, {DWIM}.