According to IATA, dangerous goods are defined as "articles or substances which are capable of posing a risk to health, safety, property, or environment and which are shown in the list of dangerous goods in the dangerous goods regulations or which are classified according to criteria specified in the United Nations (UN) Model Regulations."
In these regulations the words "shall" and "must" are used to indicate a mandatory requirement. The words "should" and "may" indicate a preferred requirement. Also, the terms "hazardous materials" and "dangerous goods" are synonymous. Since this course focuses on IATA regulations we will use the term "dangerous goods" throughout this lesson.