Often synonymous to anti-foundationalism, non-essentialism in philosophy is the non-belief in an essence (from Latin esse) of any given thing, idea, or metaphysical entity (e.g. God). Non-essentialism might also be defined cataphatically as the belief that for any entity, there are no specific traits or ground of being which entities of that kind must possess in order to be considered "that entity."
Non-essentialism is not restricted to simple philosophical speculation. It is also found in academic disciplines such as sociology, anthropology, theology, history/historiography and science. How non-essentialism is used in these discourses varies a bit given their different content and subject matter.