Ecocriticism emerged under the name 'literary ecology' in the 1990s as a way of investigating environmental crisis through the intersection between literature, culture and our physical environment. It has since developed into a literary and cultural theory. Ecocriticism remains difficult to define, as an interdisciplinary approach often broadly applied to any humanities research which addresses ecological issues. However, the founding principles of the discipline aimed towards a kind of criticism based in ecological thinking, with a moral responsibility to engage with activism. The field has developed from its first wave, a largely dehistoricised celebration of nature through environmental writing, through a second wave which deconstructed human-centered scholarship to raise more political questions of agency, imperialism and ecological degradation. We are now in what is categorised as the third wave of ecocriticism, which is defined by the movement away from an Anglo-American focus towards a global approach in line with the scale of issues such as global warming.
A more detailed explanation of this critical field, as well as links to some key introductory texts, can be found on the Oxford Bibliographies website, linked above.