DefinedTerm as well as DefinedTermSet should have the about property.

Currently, the about property is defined on the following objects:

• Certification
• CommunicateAction
• CreativeWork
• Event

The definition of DefinedTerm is the following:

A word, name, acronym, phrase, etc. with a formal definition. Often used in the context of category or subject classification, glossaries or dictionaries, product or creative work types, etc

In many cases, the defined term is about something. For instance the Bach-Werke-Verzeichnis codes are used to represent music pieces by Johann Sebastian Bach. And would be defined in the following way:

 {
    "@context": "https://schema.org/",
    "@type": "DefinedTermSet",
    "name": "Bach-Werke-Verzeichnis",
    "about": {
        "@type": "Thing",
        "@id": "https://schema.org/MusicComposition"
     }
    "hasDefinedTerm": {
        "@type": "DefinedTerm", 
        "termCode": "BWV 565",
        "about": {
            "@type": "MusicComposition",
            "name": "Toccata and Fugue in D minor",
            "genre": "Organ Work"
        }
    }
}