To me a cult is any organization that has these qualities:
1) Demands that members make unreasonable personal sacrifices. Particularly financial.
2) Leaders cannot be questioned or debated. Clear power imbalance between leadership and followers.
3) Dissuades members from associating with members outside the organization. Labels anyone who is outside the organization as evil. Members often encouraged to break family bonds (goes along with 1)
4) Members strongly discouraged from leaving (goes along with 3). Ex-members often face severe repercussions from the organization.