Articles or substances that pose a risk to health, safety, property or the environment in transport, and are therefore regulated: assigned a UN number, a hazard class, and specific packing, marking and documentation rules. "Dangerous goods" is the sea/air term; US ground calls the same freight "hazardous materials" (hazmat). Lithium batteries, machinery with fuel systems, aerosols, paints and many everyday industrial products all fall under DG rules — often to shippers’ surprise.
Full guideWhat is Class 9, and why so much freight ends up in it →