This word is used in two contexts: the relative and the ultimate. In relative terms, a dharma's dharma nature is its characteristics: the relative dharma nature of fire is that it is hot and that it burns, for example. In ultimate terms, dharma nature is the emptiness of phenomena: the ultimate dharma nature of fire is its emptiness of any true existence where there is nothing that is hot nor anything that is burning. It is this latter, ultimate sense that is most frequently used.
Although the Sanskrit word dharmata is frequently used in English texts, the translation dharma nature seems more informative to English speakers. It is important to use a compound on the word dharma to point out the relations between dharma, dharma base, dharma nature, and dharma expanse (chos, chos can, chos nyid and chos kyi dbyings). DKC