Maverick Philosopher (Bill Vallicella) offers a provisional definition of the alt-right:
I am not exactly sure what ‘alt-right’ refers to, and apparently those who fly this flag don’t either, … but I get the impression that the position includes some very specific theses that differentiate it from other types of conservatism. I hope to go into this in more detail later, but for now I’ll mention the following: white tribalism, anti-semitism, rejection of classically liberal notions such as the value of toleration, rejection of the formal (as opposed to empirical) equality of persons and with it key elements in the documents of the American founding as well as in the Judeo-Christian tradition, and a rejection of the normative universality of truth and value.
White tribalism. Tribalism is an ineluctable fact of human nature. Upper-middle-class tribalism, for example, connotes certain cultural preferences (e.g., standard English, non-violent music, education as a desideratum, a strong work ethic, marriage before children, a preference for standard literary forms). It is a rare black person who shares upper-middle-class cultural preferences, so upper-middle-class tribalism is white tribalism by default. The same can be said about white tribalism in general, that is, whites generally don’t share the cultural preferences of blacks and are therefore unlikely to associate with them on social occasions that aren’t work-group related. But the same can also be said of blacks and Hispanics of all classes. Tribalism, in short, is neither here nor there when it comes to defining the alt-right.
Anti-semitism. This seems to be rampant among non-Jewish leftists, and many leftists who are of Jewish lineage. Anti-semitism, like white tribalism, is neither here nor there when it comes to defining the alt-right.
Toleration. Persons of the left seem to display a stunning lack of tolerance for people who don’t share their political preferences (e.g., abortion, socialized medicine, heavy regulation). Neither here nor there.
Formal equality of persons. I take this to mean that everyone — or every citizen — should be accorded the same legal rights. But it’s not that simple. Legal rights today include the right to bear arms and the right to an abortion. I imagine that a person of the alt-right (in addition to most traditional conservatives and some libertarians) would uphold the first and wish to deny the second. Persons of the left, the left-center, the center, and even the center-right would, to varying degrees, restrict the first and uphold the second. My point is that almost no one believes in formal equality because there are deep divisions about the rights that should be held by citizens. Neither here nor there.
Normative universality of truth and value. Moral relativism abounds across the political spectrum, as do disagreements about what constitutes truth (e.g., the extent of and danger posed by anthropogenic global warming). I doubt that a person of the alt-right is any more prone to magical thinking than a large segment of the left. Neither here nor there.
Where does that leave me? Still wondering what defines the alt-right.