Functional Programming: what is an “improper list”?

Question

Could somebody explain what an \"improper list\" is?

Note: Thanks to all ! All you guys rock!

Answer 1

I think possibly it refers to a "dotted pair" in LISP, e.g. a list whose final cons cell has an atom, rather than a reference to another cons cell or NIL, in the cdr.

EDIT

Wikipedia suggests that a circular list also counts as improper. See

http://en.wikipedia.org/wiki/Lisp_(programming_language)

and search for 'improper' and check the footnotes.