Strictly, a variable used in metasyntax, but often used for any 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.
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. 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 qux before quux.
bazola, ztesch: Stanford (from mid-'70s on).
foo, bar, thud, grunt: This series was popular at CMU. Other CMU-associated variables include ack, barf, foo, and gorp.
foo, bar, fum: This series is reported to be common at Xerox PARC.
fred, barney: See the entry for fred. These tend to be Britishisms.
toto, titi, tata, tutu: Standard series of metasyntactic variables among francophones.
corge, grault, flarp: Popular at Rutgers University and among GOSMACS hackers.
zxc, spqr, wombat: Cambridge University (England).
shme: Berkeley, GeoWorks, Ingres. Pronounced /shme/ with a short /e/.
foo, bar, zot: Helsinki University of Technology, Finland.
blarg, wibble: New Zealand
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.