What is a cult?

In an opinion in Jacobellis v. Ohio 378 U.S. 184 (1964); Supreme Court Justice Potter Stewart famously wrote: “I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description [“hard-core pornography”]; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that.” Cults are rather like that. I know one when I see it, but how do I give words to a definition?

How would you define the word “cult” so that someone unfamiliar with the term could, given your definition, identify one if they came into contact with it? A question like this was asked several years ago, but it carried a lot of specifically sectarian baggage I most assuredly do not want associated with our discussion here. I just want to know what the word means to each of you who choose to answer.