red_flag
1. [ noun ] (psychology) the emblem of socialist revolution
Related terms: emblem
2. [ noun ] a flag that serves as a warning signal
Examples:

"we didn't swim at the beach because the red flag was up"

Related terms: flag alarm
3. [ noun ] something that irritates or demands immediate action
Examples:

"doing that is like waving a red flag in front of a bull"

Related terms: annoyance