Thermal voltage when applied AC voltage
I found the answer, and it really is what @jonk posted in the comments, it's the derivative of V(I). When I saw this I promptly remembered I saw the appendix on Malvino (bibliography of electronics course) something very similar.
So I went there again to recheck and now it's very clear and meaningful.
There's also the great trick to reuse equation B-2.

Image taken from Malvino and Bates, 8th edition Appendix B.