Hazardous means dangerous or likely to cause harm. A hazardous chemical might burn your skin or poison you if you breathe its fumes. A hazardous road has sharp curves, ice patches, or poor visibility that make accidents more likely. A crumbling staircase in an old building could be hazardous because someone might fall through the rotted wood.

You'll often see hazardous paired with specific dangers: hazardous materials (like toxic chemicals), hazardous conditions (like a thunderstorm during a hike), or hazardous waste (like nuclear material that stays dangerous for thousands of years). Scientists and safety inspectors use the word to warn people about real threats that require caution or special handling.

When something is hazardous, dealing with it requires caution. A hazard is the dangerous thing itself: a slippery floor is a hazard, and crossing it carelessly is hazardous behavior.