The Part Definition is the EZ-MES Objects that serves as a templates (blue print, hence the blue color of the icon) for Part Records. Each Part Record within EZ-MES is connected to a Part Definition, Part Record cannot exist without a Part Definition.
Part Definitions are not Parts, they are a definition of a part, and Part Records are the actual instances of these Part Definitions that represent Real-World Parts.
AKA: Part Definitions are also known as Part Types.