- a warning flag
- Informal something serving as a warning of trouble or danger ahead
- Informal something that arouses anger, irritation, revolt, etc.
A red flag serves as a warning.
The definition of a red flag is something that gives a warning of trouble.
An example of a red flag is when a series of numbers don't add up which could be because one of the numbers is listed incorrectly.
transitive verb-·flagged′, -·flag′ging
to mark, designate, or otherwise call attention to as being risky, unsuitable, etc.