Borrowing from formal grammar, you may be interested in using subject. Quoting from Capital Community College Foundation:
The subject of a sentence is the person, place, thing, or idea that is doing or being something.
For an example, consider:
That is a really pretty painting. It is simply sublime.
The painting is the subject of the sentences; it is the thing being described by the speaker. In the phrasing of the quoted source, the painting is being pretty and sublime.
Another term that could apply is object. In contrast to the subject of a sentence, the object is the recipient. Formal English grammar further breaks it down into direct objects
Art critics praised the painting as very pretty.
* Subject: art critics
* Direct Object: the painting
* Object Complement: as very pretty
and indirect objects
The art critics said "truly sublime" as they described the painting.
* Subject: The art critics
* Direct Object: "truly sublime" - an adjectival phrase, quoted, turning it into a noun.
* Linking verb: as they described
* Indirect object: the painting
Summarizing the differences (again quoting from CCCF)
A direct object is the receiver of action within a sentence.
The indirect object identifies to or for whom or what the action of the verb is performed.
In both cases, the definition of object applies to your question.
object [n. ob-jikt, -jekt; v. uh b-jekt]
2. a thing, person, or matter to which thought or action is directed:
an object of medical investigation.
As Jim Reynolds points out in the comment, these words are fairly broad. They are hypernyms of your specific example. It seems that the askers own find of "Descriptum" is the most accurate, most specific word. Nevertheless, subject and object remain well understood parts of speech. The sentence "This painting is the subject/object of the description in question." is perfectly understandable. Whether subject or object is more correct depends on the grammatical voice; in other words, the phrasing of the description itself.