The term was coined by Eric Sink in late 2004 to mean software "companies made up of exactly one person".
Revenue is not a factor - as pointed out in the link above, some MicroISVs have made at least $1 Million.
I don't think people strictly adhere to the "one person" definition though. Even in Eric's initial post, some of the people mentioned actually work with their spouse, or even outsource some of their coding work. It's further complicated by some governments and organisations defining a "micro enterprise" as having 5 or fewer employees.
- Its primary distribution channel is the Net. Not retail stores. Therefore its revenue/cost structure is very, very different from a company that needs to fight for shelfspace.
- It is largely self-funded. There may be some angel investor money in there, but you are not playing with VC money, using VC rules. Nothing wrong with VC money, but that's your "traditional" startup.
- Very few (1-5, 6 maybe 7) people are owners/employees. Group/management dynamics change when you go from 5 to, say, 20 people. Again, nothing wrong with that, but it's a different kind of company.
Most MicroISVs today would have been categorized as shareware authors in the 80s or 90s.
Are there any recent statistics about this? I bet that there are a lot more one man shops around which subcontract the skills they don't have rather than multi-employee companies.
MicroISVs can be seen as just as a professionalization & industrializaion of the shareware industry, the Internet has helped tremendously to leverage this.