What's the "een" in Halloween?
A software life-cycle model which supposes incremental development, using the waterfall model for each step, with the aim of managing risk. In the spiral model, developers define and implement features in order of decreasing priority.
[Barry Boehm, "A Spiral Model of Software Development and Enhancement", ACM SIGSOFT Software Engineering Notes, August 1986].
[Barry Boehm "A Spiral Model of Software Development and Enhancement" IEEE Computer, vol.21, #5, May 1988, pp 61-72].