Silicon Valley Business School
Mezzanine Stage Milestones 
Milestones and Stepping Stones in the Mezzanine Stage of a Startup's Development

According to Wikipedia, a mezzanine is an intermediate floor between main floors of a building; it is often low-ceilinged, and often projects in the form of a balcony.  In the technology startup business, a mezzanine-stage company is an intermediate phase that the Company goes through after it has successfully launched and established an expanding business but before it launches onto the public markets through an IPO.  For many companies, the mezzanine stage is the time to prepare for the transition from privately held to a publicly held corporation.  This is often the first time that the Company has to deal with Wall Street investors and SEC (Securities Exchange Commission) regulations.   Milestones for mezzanine stage companies often include taking all the steps involved in the IPO process.

Notes about Milestones
Milestones have to be achieved in parallel—when reviewing these milestones, it's important to recognize that the process is not entirely serial. Management has to work on several milestones in parallel. Milestones are different for every company—the list of milestones has to be tailored for each company to reflect its specific market, technologies and challenges. Milestones have dependencies—some milestones cannot be commenced until others have been successfully completed.