What is a cult exactly?

How is the word "cult" defined? Webster's Dictionary defines a cult as: "1. A formal religious veneration 2. A system of religious beliefs and rituals also its body of adherents; 3. A religion regarded as "unorthodox or spurious."; 4. A system for the cure of disease based on dogma set forth by its promulgator; 5. a: A great devotion to a person, idea, thing; esp.: such devotion regarded as a literary or intellectual fad, b: A usually small circle of persons united by devotion or allegiance to an artistic or intellectual movement or figure." This definition obviously could include everything from Barbie collectors to old "Deadheads," "Trekkies" to diehard Elvis fans. American history might also include within such a definition the devoted followers of Mary Baker Eddy the founder of Christian Science, or the Mormons united through their devotion to Joseph Smith. Both these religious groups were once largely regarded as "unorthodox or spurious." However, the most important concern today is not simply who might be somewhat "cultic" in their devotion now or historically, but what groups might represent potential problems regarding personal or public safety. That is, groups that are potentially unsafe and/or destructive. Psychiatrist Robert Jay Lifton, who wrote the definitive book about thought reform (often called "brainwashing") also wrote a paper about cult formation. So what is a cult according to Lifton? He defined a cult as having the following three characteristics: 1. A charismatic leader, who increasingly becomes an object of worship as the general principles that may have originally sustained the group lose power. 2. A process [is in use] call[ed] coercive persuasion or thought reform. 3. Economic, sexual, and other exploitation of group members by the leader and the ruling coterie.