What does the yellow diamond on the NFPA 704 fire diamond represent?

The story behind the answer

The yellow quadrant of the NFPA 704 “fire diamond” indicates a material’s instability/reactivity—how prone it is to detonate, decompose violently, or react dangerously (often with water or air). It’s rated 0–4, from stable (0) to may detonate (4), helping first responders gauge explosion risk. This is different from red (flammability) and blue (health); radioactivity is not covered by NFPA 704. The white quadrant lists special hazards (e.g., OX for oxidizer, W for reacts with water).

Historically labeled “Reactivity,” the current NFPA terminology is “Instability,” so many references use “Instability (Reactivity)” to avoid confusion.