Most scholars of religion have abandoned the word "cult" in the senses that it came to be used a couple of decades ago and on occasion still is. When the Unification Church, the Divine Light Mission, the Hare Krishna group, and scores more were attracting especially the offspring of well-off suburbanites, nonplussed parents and others were threatened, and needed a term to describe the movements that produced the threats. They reached back for "cults," and for some years a stream of books appeared with the word 'cult' on the cover.
Where draw the line between a cult and everybody else? It became clear that the word was almost always used pejoratively: "we" good people are secular or we belong to a standard-brand denomination, church, temple, mosque, while "you" bad people brainwash others and do strange and secret and scary things. Again, where was the borderline? To anti-Catholics, the formation of monks and nuns was cultic; to anti-Baptists, becoming "born again" meant entering a cult. Eventually it became clear that everyone called someone a cult, and the word served few clarifying purposes.
Earlier the word had been "sect," a term which began innocently but came to be used to dismiss others. 'Sect' was less condemnatory than "cult," but it still wasn't nice. To this day some headlines will refer to a mainline church as a sect, but wins no points for doing so.
When words get tired and nothing-but-misused or confusing, people who like to use words with conceptual priority and good manners shelve them. The usual and most nearly neutral term was "New Religious Movements," NRMs, which worked for those that were new. Of course, many claim ancient ancestry, but are simply "new to us." If we need an all-purpose categorizing term, the NRM concept will have to serve. "Denomination" works for those that become mainstream, but not all in them like to be called that, either. All categorizing and cataloguing terms have to be handled with care.
In his Varieties of Religious Experience, William James noted that in order to understand phenomena we must catalogue and label and categorize, but must also know that we miss something. Thus, if you could interview a crab, he said, the creature would protest being categorized as a crustacean: "I am not a crustacean. I'm a crab. I'm not even only a crab; I want to be taken for MYSELF, MYSELF." Give his or her SELF a chance to define herself, is the best advice.
Meanwhile, goodbye "cult." It's been non-nice to know you.