The thing is, if you use the term "Startup" here to describe anything other than a zero-profit 80hr/week scramble for VC funding and eventual acquisition, you're going to get people popping up and claiming that what you're doing is not startuppy enough to count. So for the benefit of that crowd, you might want to rephrase your question in terms of building a product that brings in enough revenue to quit your job.
If that's what you want to build, then yes. It's absolutely possible, and there are dozens of people here who have done so.