Are you asking about the referent of the word "nirvana"?
The referent of the word nirvana is a concept of nirvana, and the referent of that concept is ... WHAT?
The referent of that concept is the actual experiential liberation of the mind.
Liberation from what? Liberation from the confines of concepts and therefore from the dukkha that comes from the clash between concepts, or between the concepts on one hand and the ontological reality on the other.
Now you tell me, is this Liberation a conceptual construct? For you perhaps it is, because you are asking about it? For me - not anymore, because I attained it in practice.
Once attained, the concept of nirvana has no referent. One does not engage in semiotic games anymore. No bullshit like "nirvana" and "liberation".