No, there isn’t really a definition, as it’s more a collective term for prevalent ideologies and pieces of artwork produced in a certain era from the aftermath of the second world war onwards (circa 1950s-1980s).
It’s definitely true to say that both modernism and post-modernism sought to challenge hegemony, but postmodernism
wasn’t an extension of modernism, but an active retaliation against the elitism, optimism and authoritative leanings of modernism.
Modernism attempted to legitimate and protect ‘high culture’, whilst disparaging the ‘low culture’ of the masses. Eugenics, for instance, was widely embraced by many modernists. Furthermore, modernist art and literature was highly reverent of tradition (see: T.S Eliot’s essay ‘Tradition and the Individual Talent’, and James Joyce reworking of Homer’s the Odyssey in
Ulysses). The most prominent modernists were all extremely well educated, relatively wealthy, and white.
Alternatively, postmodernism is much more inclusive and it embraced popular culture. It extended into various art forms: household objects, interior design, fashion, music videos, photography etc. Importantly, it was consciously commercial and most postmodernist art was mass produced, which meant that anyone could consume art (rather than a select, privileged few). It was mean to be fun and free (though was of course often sceptical in tone) and you’ll notice a lot more vibrant colours and bricolage used in postmodernist art.
(Modern day) hipsters aren’t postmodernist, as whether they’re aware of it or not, they are concerned with the relentless acquisition and therefore reification of sub-culture, which is something else entirely – possibly post-postmodernism?
I attended a wonderful exhibition on Postmodernism at the V&A last autumn which helped solidify my understanding of what Postmodernism is – sometimes it’s better to actually look at examples of work produced in the postmodernist period instead of grappling with definitions.
P.S - this is one of my favourite postmodernist pieces of art, which I have hanging in my room: