What software development model is shown here?

The illustration shows the spiral model of software development. In this approach, developers use multiple iterations of a waterfall-style software development process. This becomes a loop of iterations through similar processes. The original waterfall approach does not iterate through the entire process repeatedly. Some variants do allow iteration, but only by allowing movement backward and forward one stage. The Agile approach to software development focuses on iterative improvement and does not follow a rigorous SDLC model. Lean is a process improvement methodology and not a software development model.