The main difference between Effectuation and the Lean Startup is its origins: the Lean Startup was a term used in Academia, whereas Effectuation comes out of Silicon Valley practice.

While both streams describe entrepreneurial search as an iterative process, the effectuation stream suggests that success comes only after 2 cycles of test-and-learn, whereas the Lean Startup suggests 5 cycles.

There is no difference between Effectuation and the Lean Startup. Both concepts can be used interchangeably. 

While both streams describe entrepreneurial search as an iterative process, the focus of the Effectuation stream is to understand how the entrepreneurial search helps a person find a solution, whereas the Lean Startup is more focused on the success of the project.