The definition of Dharma is essential occupation or relation.
The definition of Jiva is living being
The word Jaiva means of or related to the Jiva.
Therefore, Jaiva-Dharma means the dharma of the Jiva, or the characteristic function related to the Jiva.
The author answers this question exhaustively in this book, but I still think that is it is essential to submit one or two point in brief.
The word Jivana (life) comes from the word Jiva, wich means ‘one who have life.’
In other words, all living beings are known as Jiva.
thus, the author has used the term ’jaiva-dharma’ to indicate
the constitutional function of the Jiva.
Jivera svarupa haya krsnera nitya das
Krsnera tatastha- sakti bhedabheda prakasa
The Jiva’s natural condition is to be a servant of Krsna.
The Jiva is the marginal potency of Krsna, and a manifestation which is both one with and different from Krsna.
Reverence Sri Caitanya Caritamrta, Madhya (20.108)