any person who advocates or works to protect the air, water, animals, plants, and other natural resources from pollution or its effects.
a person who believes that differences between individuals or groups, especially in moral and intellectual attributes, are predominantly determined by environmental factors, as surroundings, upbringing, or experience (opposed to hereditarian).
1923, as a psychological theory (in the nature vs. nurture debate), from environmental + -ism. The ecological sense is from 1972. Related: Environmentalist (n.), 1916 in the psychological sense, 1970 in the ecological sense.