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.

Metasyntactic variables are so called because (1) they are variables in the metalanguage used to talk about programs etc; (2) they are variables whose values are often variables (as in usages like “the value of f(foo,bar) is the sum of foo and bar”). However, it has been plausibly suggested that the real reason for the term “metasyntactic variable” is that it sounds good. To some extent, the list of one's preferred metasyntactic variables is a cultural signature. They occur both in series (used for related groups of variables or objects) and as singletons. Here are a few common signatures:

foo, bar, baz, quux, quuux, quuuux...: MIT/Stanford usage, now found everywhere (thanks largely to early versions of this lexicon!). At MIT (but not at Stanford), baz dropped out of use for a while in the 1970s and '80s. A common recent mutation of this sequence inserts quxbefore quux.
bazola, ztesch:Stanford (from mid-'70s on).
foo, bar, thud, grunt:This series was popular at CMU. Other CMU-associated variables include gorp.
foo, bar, bletch: Waterloo University. We are informed that the CS club at Waterloo formerly had a sign on its door reading “Ye Olde Foo Bar and Grill”; this led to an attempt to establish “grill” as the third metasyntactic variable, but it never caught on.
foo, bar, fum:This series is reported to be common at XEROX PARC.
fred, jim, sheila, barney:See the entry for fred. These tend to be Britishisms.
flarp:Popular at Rutgers University and among GOSMACS hackers.
zxc, spqr, wombat:Cambridge University (England).
shmeBerkeley, GeoWorks, Ingres. Pronounced /shme/ with a short /e/.
foo, bar, baz, bongoYale, late 1970s.
spam, eggsPython programmers.
snorkBrown University, early 1970s.
foo, bar, zot Helsinki University of Technology, Finland.
blarg, wibbleNew Zealand.
toto, titi, tata, tutuFrance.
pippo, pluto, paperinoItaly. Pippo /pee�po/ and Paperino /pa�per�ee'�no/ are the Italian names for Goofy and Donald Duck.
aap, noot, miesThe Netherlands. These are the first words a child used to learn to spell on a Dutch spelling board.
oogle, foogle, boogle; zork, gork, borkThese two series (which may be continued with other initial consonents) are reportedly common in England, and said to go back to Lewis Carroll.

Of all these, only foo and bar are universal (and baz nearly so). The compounds foobar and foobaz also enjoy very wide currency. Some jargon terms are also used as metasyntactic names; barf and mumble, for example. See also Commonwealth Hackish for discussion of numerous metasyntactic variables found in Great Britain and the Commonwealth.