An investor who is on the wrong side of a stock for an extended period of time, with the consistent belief that he or she is in fact correctly predicting its direction, despite strong evidence to the contrary. Typically, this person loses most or all of his or her money in the process.
The term comes from people in soup lines during the Great Depression, who held potato bags filled with their only possessions.
"All the retail investors who bought priceline.com in early 2000 saw their investment value shrink 99% over the following year. They were the bag holders the smart traders unloaded their shares onto."
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!"