It's a pretty vague term and can change based on the context in question, but basically it's an Object that represents and/or abstracts and/or encapsulates a set of Data.
IBM - What Is a Data Object? (DB context)
Data objects ... represent an object view of related tables.
MS - DataObject Class (MS .Net context)
defines a format-independent mechanism for transferring data
After reading th example you gave for context we need to clarify something.
There are Object-Based languages, and then there is Object-Oriented programming. The procedural language you are reading about there is an Object-based language, but doesnt' fully support OO programming. Meaning it allows "objects" to be created, but OO-specific stuff (like polymorphism) doesn't exist.
Having said that, my original statement still stands, but I'll add a bit to it.
A "Data Object" is an object that encapsulates both the data itself (in object properties, similar to "variables", in that they are a memory segment defined as a certain data type (string, integer, object, etc.), as well as the methods (procedures) for accessing said data (in an abstract way).
From a page back from your provided context page:
Object-based languages merge the two together by encapsulating the procedures and the data together in an object.