It's the minimum set of requirements to have a fully functional, usable product.
The MVP has to be customer-ready, so it has to be as minimally feature-complete as possible.
It's the minimal setup that allows you to test a hypothesis about a proposed product or an existing product's next iteration.
Whatever works that demonstrates an idea of something that can be achieved.
I would argue the MVP is what you can sell to customers. And if your business is going to succeed, your MVP arguably needs to be something that is not just good enough to sell to customers, but which customers will come back for more when you're ready to launch the next thing.
It's usually the product with the smallest set of features that can show whether there is going to be traction with the offering.