Class 3 hazardous materials are defined by their:

Class 3 hazardous materials are specifically defined by their flash point, which is the lowest temperature at which vapors of the material can ignite when exposed to a flame or spark. This characteristic is crucial for identifying flammable liquids, which fall under Class 3. Understanding the flash point helps in evaluating the potential fire hazards associated with a material. It is significant for safety regulations, transportation guidelines, and emergency response planning.

The other options, while they may describe properties of hazardous materials, do not serve as the defining characteristics for Class 3. Density refers to the mass per unit volume, viscosity describes a fluid's resistance to flow, and boiling point indicates the temperature at which a substance changes from liquid to gas. However, none of these properties determine the classification of a material under Class 3 hazardous materials.