Dangerous Goods: Articles or substances which are capable of posing a significant risk to health, safety or property when transported by air and which are classified according to the U.S. DOT and IATA/ICAO List of Dangerous Goods.
Hazmat Certification, DG Certification, DG Certified, Hazmat Certified etc. all refer to the concept that someone, some entity is conferring approval on an individual or company that they are authorized to ship hazmat in commerce.—Unfortunately, there’s a lot of confusion and misinformation in this area.—In the US, only the employer can certify employees. The DOT does not certify anyone; never has.
Definition Source: http://www.dgtraining.com/Resources/Glossary.htm