An investor who trades shares on an exchange through a third party, typically a brokerage firm with direct access to the market(s) ECN networks. This term is often used in a derogatory sense to describe various middle-of-the-road people who attempt to amass great wealth with little to no research beforehand, often resulting in the complete loss of their "invested" money.
Being a retail investor is not a sign of incompetence, however; there are many who are quite able and consistently beat indices and funds with their returns.
Noob trader: "Hey! This stock just fell 50%; there's gotta be a short squeeze soon! I'm buying!"
Experienced trader: "Don't be a retail investor, man!"
Experienced retail investor: "Bag holder would be a better phrase for that, buddy!"