A 2019 research paper, The Upsurge of Irrationality; Post-truth politics for a polarized world, discusses how researchers see the recent descent of political discourse into the mess it is today for tens of millions of Americans. It nicely describes what concepts such as post-truth mean and how they can influence thinking and political and social policy. The following are some quotes from the paper.
.... the term “truthiness”, coined in 2005 by the comedian Stephen Colbert and defined as “the quality of seeming or being felt to be true, even if not necessarily true.” So, truthiness is not necessarily falsehood or propaganda: it can be mere ignorance shaped by emotion, “gut feeling” and overreliance on intuitive thinking. Nevertheless, while truthiness was used primarily for political satire .... post-truth is not a joke any more.
Current social polarization has led to an upsurge of collective irrationality in which formerly underground unwarranted beliefs and radical discourses have become mainstream. .... controverted shared values have been replaced by alternative epistemologies shaped by identity-related empirical misconceptions that are at the core of current cases of “culture war.” This state of affairs has recently been called “post-truth.”
There are several interconnected concepts considered as major forms of collective irrationalism, such as pseudoscience, science denialism, fact resistance, and alternative facts. Post-truth has emerged as a higher-order concept that describes the current sociological state of affairs in which all these forms of irrationality thrive. This recent term is defined as “relating to or denoting circumstances in which objective facts are less influential in shaping public opinion than appeals to emotion and personal belief.”
The meaning of post-truth goes beyond being a fool or a liar — “in its purest form, post-truth is when one thinks that the crowd's reaction actually does change the facts about the lie (...) what seem to be new in the post-truth era is a challenge not just to the idea of knowing reality but to the existence of reality itself.” In this regard, although political lies have always existed, “post-truth relationship to facts occurs only when we are seeking to assert something that is more important to us than truth itself. Thus, post-truth amounts to a form of ideological supremacy, whereby its practitioners are trying to compel someone to believe in something whether there is good evidence for it or not.” So, while truthiness locates the responsibility for lying, post-truth is more vague and collectivist in this regard, providing no clear way to define who is responsible, when, and to what extent. Hence, post-truth gives rise to “a world in which politicians can challenge the facts and pay no political price whatsoever.”