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

In Common Lisp improper lists are defined as:

• dotted lists that have a non-NIL terminating 'atom'.

Example

  (a b c d . f)

or

• circular lists

Example

  #1=(1 2 3 . #1#)