Formal definitions aside, accessibility means a product is available to everyone regardless of the way they operate their device.
Usability guarantees the product is easy to use, intuitive and easily learnable.
User experience on the other hand tries to make a product attractive and fun – turning it into an experience rather than just another mundane tool.
I’ve tried and failed to order these terms into a nice, logical pyramid-like structure to visualize the importance and hierarchy between them. Besides, half of the UX designers out there would have probably disagreed on the hierarchy anyway ;).
For maximizing a product’s audience, the most sensible approach would be to first make sure the product is accessible to all, then work on it’s usability to support the actual functionality we’re trying to sell and finally package the whole thing in a compelling user experience design.
Ironically, even though from my perspective product accessibility is a “must” and great user experience would only count as a “nice-to-have”, the most basic design decisions such as who an app is for and what it should actually do are questions of unmet needs which should be answered by a UX researcher before anybody even does a line of coding or design work.
Of course, you can decide to ditch accessibility and make a pretty, shiny and totally inaccessible product which will give your young, lean and always energetic audience thrills along their spines and never mind those aging grannies that you’re probably discouraging from ever even looking at your new invention, why not? It’s a free market.
But keep in mind that accessibility often turns out to have much in common with ergonomics and will also make it easier for your regular audience to effectively use the product.