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.
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).
zxc, spqr, wombat: Cambridge University (England).
foo, bar, zot: Helsinki University of Technology, Finland.
blarg, wibble: New Zealand
See also Commonwealth Hackish for discussion of numerous metasyntactic variables found in Great Britain and the Commonwealth.