A social and political movement focused on protecting and improving the natural environment.
From 'environment' plus '-alism' (a suffix denoting a movement, belief system, or practice). The modern word emerged in the 1960s-70s as ecological concerns became a social movement.
'Environmentalism' only became a common word when the modern environmental movement took off in the 1970s! It's a perfect example of how new social movements create new words to describe themselves.
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