The way I define Fullstack is that it is not limited to frontend and backend work. It’s closer to the hardware-engineer argument: can one person actually build an entire computer? Maybe not every part from scratch but a capable engineer can assemble the pieces into a working system.
By that logic, a full-stack engineer is someone who can pull together everything needed to turn an idea into a product. I measure their skill by how fast and effectively they are able to deliver: design, engineering, requirements, and even a bit of SEO when the product calls for it.
Where I separate a full-stack engineer from a product engineer is in the focus.
A full-stack engineer focus is almost entirely technical - think optimizing page speed, bundle sizes, etc. A product engineer is maybe 70% technical, but adds extra 30% of domain thinking - think competitor analysis, customer empathy, and product sense. A product engineer is the kind of person you would actually put in front of customers.
From what I see around me, while most everyone seems to say that this is a good thing, in practice almost everyone in such a position is seeking to move to either a management role or a specialist role, or just to launch their own business, at which point they're by definition an entrepreneur rather than an engineer.