This is an assumption that the product or service will appeal continue to grow to a sustainable business with enough customers that is not only a very small niche.
This does not mean niche’s are bad, but that your niche is at least big enough and will grow into that niche.
Another important assumption at the start is The value hypothesis.
Reference
Ries, Eric. The Lean Startup: How Today’s Entrepreneurs Use Continuous Innovation to Create Radically Successful Businesses. 1st ed, Crown Business, 2011.
Highlights or timestamps
The growth hypothesis states that the product will not only appeal to the small group of early adopters but will also find a bigger market later.
— ^aaf493 from The Lean Startup