Someone with a stop-loss order doesn't seem like a retail investor to me.
Retail investors buy and hold till retirement.
Retail investors buy and hold till retirement.
Not for frequent trading as a mechanism to proxy the market.
Trading and investing are completely different animals, which you as a trader know very well.
Typically a "retail investor" is thought of as someone who uses a retail broker. That is, as opposed to using an IBank's trading desk, an internal trading platform, bloomberg, etc.
http://lexicon.ft.com/Term?term=retail-investor
EDIT: This is the definition used by the FINRA (Financial Industry Regulatory Authority):
“Retail investor” means any person other than an institutional investor, regardless of whether the person has an account with a member. “Institutional investor” includes a bank, savings and loan association, insurance company, registered investment company, SEC or state registered investment adviser, natural person or entity with total assets of at least $50M, government entity, certain employee benefit plans with at least 100 participants (but not any participant of such plans), FINRA members and their registered persons, and any person acting solely on behalf of an institutional investor.
At the end of the day these terms largely are undefined and used loosely. Is a day trader with a TT terminal in their home a "retail investor"? If so, some of the biggest movers in options markets in the world are "retail investors".
The needs of that person vs someone who trades a couple of times a month via Robinhood are clearly much different.
http://lexicon.ft.com/Term?term=proprietary-trading
I don't see why an individual day trader (even if he has a TT terminal, whatever that might be, at home) wouldn't be considered a "retail investor".
Again, these terms are loosely defined in the industry and the only reason it is important is because of the behavioral expectations.
If you trade all day, whether with your own money or someone else's, on professional trading platforms (which TT or trading technologies is a very commonly used example) you should be held to different standards than someone who occasionally buys and sells shares via etrade. It is your job at that point to understand precisely how stop loss orders work and no one should give you sympathy when you do your job poorly.
If we aren't going to use "retail investor" to distinguish between those 2 use cases thats fine, but we need a new terminology in that case.
Google's own definition:
"A retail investor is an individual who purchases securities for his or her own personal account rather than for an organization. Retail investors typically trade in much smaller amounts than institutional investors such as mutual funds, pensions, or university endowments."
FINRA clearly has a different definition than google. The google definition doesn't seem to hold for high volume professional traders. You yourself said "you're a professional when you're investing other people's money."
Largely it quite simply doesn't matter what definition you describe, but when people say things like "HFT is bad for retail investors" or "Flash crashes on Monday really hurt retail investors" it is important to understand the nuance.
It is also important to understand the context. When you are sitting in an options arcade and a trader says retail investor, they are quite simply NOT referring to themselves, even though by several definitions they are retail investors. When FINRA uses retail investor in their documents they also mean something different than the google definition.