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metasyntactic variable n.
A name used in examples and understood to stand for whatever thing is under discussion, or any random member of a class of things under discussion. The word [foo] is the [canonical] example. To avoid confusion, hackers never (well, hardly ever) use `foo' or other words like it as permanent names for anything. In filenames, a common convention is that any filename beginning with a metasyntactic-variable name is a [scratch] file that may be deleted at any time.