en·vi·ron·men·tal·ist / enˌvīrənˈmen(t)l-ist; -ˌvī(ə)rn-/ • n. 1. a person who is concerned with or advocates the protection of the environment.2. a person who considers that environment, as opposed to heredity, has the primary influence on the development of a person or group.DERIVATIVES: en·vi·ron·men·tal·ism n.

environmentalist
1. One who holds that damage to the natural environment resulting directly from human activity is so severe as to present a challenge to the survival of many habitats and ultimately perhaps to the continuance of life on Earth, and can be redressed only by major reforms of the way people live and industries function.

2. One who holds that the environment exerts a more important role than heredity in determining the characteristics of individuals (i.e. favouring nurture in the debate over the relative contributions of nature and nurture).