Source : Free On-Line Dictionary of Computing

singleton variable
     
         A {variable} which is only referred to once in a
        piece of code, probably because of a programming mistake.  To
        be useful, a variable must be set and read from, in that
        order.  If it is only referred to once then it cannot be both
        set and read.
     
        There are various exceptions.  {C}-like {assignment
        operators}, e.g. "x += y", read and set x and return its new
        value (they are abbreviations for "x = x+y", etc).  A
        {function} {argument} may be passed only for the sake of
        uniformity or to support future enhancements.  A good
        {compiler} or a {syntax} checker like {lint} should report
        singleton variables but also allow specific instances to be
        marked as deliberate by the programmer.
     
        (1997-12-20)