The Free On-line Dictionary of Computing (30 December 2018):

dotted pair

    The usual LISP syntax for representing a cons
   cell that is not a list.  For example, the expression

   	(cons 'foo 42)

   returns a cons cell that is output as

   	(foo . 42)

   which represents a cons cell whose car is the symbol "foo" and
   whose cdr is the integer 42.

   (2014-11-09)