You can list it as "Unpaid Sabbatical: Application Name". Then, put bullet points for the things you were trying to accomplish with this application, what challenges you encountered and how you addressed them. Be careful not to puff it up beyond what it was, but absolutely don't shy away from the work done. How you frame the work matters -- if you list something as an actual startup, for example, I'd probe you on your customer discovery process, etc.