A better definition, and one used by wide swaths of the secular anti-cult community, is:
1 - They lie to you when you come in the door. The organisation's motives and what will eventually happen to you are not part of the presentation that gets you through that initial door and are probably specifically denied: religions deny that they are religions; political movements deny that they are political movements. Front groups abound. This is necessary to get anyone in the door and to incrementally change recruits lives into something totally fucked up.
2 - It's totalitarian. The Great and Noble Cause explains all aspects of reality with complete accuracy and every part of your life should be informed by this. Mismatches between facts and dogma are not possible and anyone purporting any such are liars and enemies of the Great and Noble Cause.
3 - There's somebody or some committee that decides all details and specific cases of what the dogma is and how it should be applied.
Note that there are many evil and disgusting organisations for whom 1 or 2 of these are true. They are not cults. In biology a similar observation is that the existence of the Black Death does not mean that malaria does not exist and that a description of a cold as a runny nose, watery eyes and a fever is not invalidated by the fact that peeling onions can make your eyes water.
Note especially that religion is not mentioned as it's not particularly useful to classify things that way. See the cults of neocons, Lydon Larouche, many Multi-Level Marketing schemes, and North Korea. Religious terminology can be transduced to: Religion - Rock and Roll is great! ; Sect - but Elvis was the best; Cult - and we can show how to live your life just like The King so that you may partake of his greatness!