Although, I would ultimately disagree with Vattimo that someone could actually be postmodern today, as the ironic turn and dissolution of history is still within the modern project, this is still one of the better definitions I have seen:
Let us therefore try to state, as clearly as possible, what it is we mean when we speak of “postmodernity,” and let us begin by addressing the ambiguity of the word itself: if “modern” is taken to mean the latest style, whatever is newer than what went before, then “postmodernity” can hardly mean anything other than the most recent, most modern modernity–precisely the opposite of what it really means. This is not simply an instance of terminological confusion. It is the logic of modernity itself that hampers us when we try to speak about postmodernity. The logic of modernity is the logic of linear time, a continuous and unitary process that moves toward betterment, in the Enlightenment vision of modernity at any rate. But even when, as in reactionary thought, the process is conceived as a road sloping toward decadence, the logic remains linear, with time as a single strand unraveling toward the worse instead of “making progress.” Now if there is one thing that constitutes the essential content of the idea of postmodernity, and also its logical possibility, it is the negation of this unilinearity of historical time. We are not postmodern because we come after modernity, nor because, having arrived later, we are farther along the road to the better or the worse We are postmodern because these dimensions, which were always temporal and axiological for modernity, no longer have meaning for us. Obviously one thing continues to happen after another, but the positioning of this succession a “a” time conceived as an ultimate and absolute dimension, as the over all horizon of meaning, no longer holds good.
Gianni Vattimo, Nihilism & Emancipation: Ethics, Politics, & Law, 49-50.