@Robindra - In any articles you write referencing the plant, you need to use the name of the plant used by the ethnic group using it. However, you also need to find the botanical name for the plant, and give that as well. It is highly unlikely that it has not been properly identified and named by a botanist. You need to find a botanist that has worked in the wider area where you located the plant to guide you to sources for botanical identification. Further, I would like to know how you are certain this plant is used by only one ethnic group. Until you know the botanical name of the plant you won't know if it is used, by a different indigenous name, by other groups.
Ethnobotany is not exactly a term for the plants themselves so much as the study of the connection between specific cultures and their plants, but perhaps it's a start? I wonder if ethnobotanists have generated a suitable term for plants of this category with the sort of relationship to an indigenous group you are describing...