Word for Thing Being Described?

The referent.

referent n.

A person or thing to which a linguistic expression refers.

American Heritage® Dictionary of the English Language, Fifth Edition.

referent n

1. the object or idea to which a word or phrase refers.

Collins English Dictionary

I did some more research actually and I found exactly what I was looking for! What I did was I looked up descriptor in the Oxford English Dictionary (OED) and looked in the "nearby words" section. Turns out that a word for it was used as early as 1933: descriptum. See clickable screenshot below.

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.

<sup>* Subject: art critics
* Direct Object: the painting
* Object Complement: as very pretty</sup>

and indirect objects

The art critics said "truly sublime" as they described the painting.

<sup>* 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</sup>

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]
noun
2. a thing, person, or matter to which thought or action is directed:
an object of medical investigation.

^{Source: dictionary.com}

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.