Aesthetic relativism is the idea that views of beauty are relative to differences in perception and consideration, and intrinsically, have no absolute truth or validity.
(A frequently-cited source for philosophical relativism in postmodern theory is a fragment by Nietzsche, entitled "On Truth and Lie in an Extra-Moral Sense".)
Aesthetic relativism is a variety of the philosophy known generally as relativism, which casts doubt on the possibility of direct epistemic access to the "external world", and which therefore rejects the positive claim that statements made about the external world can be known to be objectively true.
[1] Philosophers who have been influential in relativist thinking include David Hume, particularly his "radical scepticism" as set out in A Treatise of Human Nature; Thomas Kuhn, with regard to the history and philosophy of science, and particularly his work The Structure of Scientific Revolutions; Friedrich Nietzsche, in moral philosophy and epistemology; and Richard Rorty, on the contingency of language.
The most prominent philosophical opponent of aesthetic relativism was Immanuel Kant, who argued that the judgement of beauty, while subjective, is universal.