Postmodernism is a philosophical movement evolved in reaction to modernism

Modernism

Modernism, in its broadest definition, is modern thought, character, or practice. More specifically, the term describes the modernist movement, its set of cultural tendencies and array of associated cultural movements, originally arising from wide-scale and far-reaching changes to Western society...

, the tendency in contemporary culture to accept only objective truth and to be inherently suspicious towards a global cultural narrative or meta-narrative

Metanarrative

A metanarrative , in critical theory and particularly postmodernism, is an abstract idea that is thought to be a comprehensive explanation of historical experience or knowledge. According to John Stephens, it "is a global or totalizing cultural narrative schema which orders and explains knowledge...

. Postmodernist thought is an intentional departure from the previously dominant modernist approaches. The term "postmodernism" comes from its critique of the "modernist" scientific mentality of objectivity

Objectivity (science)

Objectivity in science is a value that informs how science is practiced and how scientific truths are created. It is the idea that scientists, in attempting to uncover truths about the natural world, must aspire to eliminate personal biases, a priori commitments, emotional involvement, etc...

 and the progress associated with the Enlightenment

Age of Enlightenment

The Age of Enlightenment was an elite cultural movement of intellectuals in 18th century Europe that sought to mobilize the power of reason in order to reform society and advance knowledge. It promoted intellectual interchange and opposed intolerance and abuses in church and state...

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Postmodernism postulates that many, if not all, apparent realities are only social constructs and are therefore subject to change.