postmodernism destabilizes other concepts such as presence, identity, historical progress, epistemic certainty, and the univocity of meaning

 That postmodernism is indefinable is a truism. 

However, it can be described as a set of critical, strategic and rhetorical practices employing concepts such as difference, repetition, the trace, the simulacrum, and hyperreality to destabilize other concepts such as presence, identity, historical progress, epistemic certainty, and the univocity of meaning