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).
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.
Edward Feser describes the position as not only untenable logically but psychologically impossible.
In his book Aristotle's Revenge he argues that one cannot say the universe essentially does not have an essence without violating the Law of noncontradiction.