Cult is not an easy word to define.
In popular culture usage, it is often used to refer to any "group whose beliefs or practices are considered abnormal or bizarre"1. This easily explains the bulk of the general usage, particularly by media. It is an easy label to slap on any group small enough if their practices are abnormal or bizarre enough to attract attention.
However I would argue that this definition is roughly meaningless in the context of Christianity.
During the period the term was first established in English, it often carried the connotation of being "new". Any new religious movement, whatever their beliefs were, could be labeled as a cult. This partially explains why faiths new to the world scene (such as Mormonism) were easily qualified as Cults at first, but that label is disputed today. In popular usage, there seems to be some expectation that cults would eventually self-destruct (e.g. Waco, Jonestown, Heaven's Gate) whereas any set of beliefs that survived the second generation should be considered a religion (or sect of a religion as appropriate) as opposed to a cult.
In the context of established religions it can have a much more specific and meaningful connotation. This definition is based on the relationship of the Cult to some other religion. The term is used this way to describe cults of other religions as well. The definition I'd suggest as most meaningful goes something like this:
A group of people who claim identity/membership with an established religion but who, because of their beliefs, are rejected by the members of that religion as being unorthodox.2
In this sense, the term is only useful when combined in reference to what religion they were originally related to. An Islamic Cult would be a group that claimed affiliation with Islam (or even claimed to be true Islam) who's views are divergent enough that are rejected by the larger main body of Islamic teaching. The same would be true for a Buddhist Cult.
Applied to Christianity, this definition would encompass any group of any size smaller than the whole who claimed to be "Christian" but were identified by more established or larger bodies as being divergent on enough core beliefs to actually make them a different religion. What beliefs qualify as core does vary depending on who you talk to, but not so much as to make it impossible to use this label in a general and meaningful way. The standard issues would be the nature of God, particularly in relation to the Trinity, and the fundamental nature of man and how salvation is accomplished.
Using this definition, it is usually accepted that the major branches that comprise mainline Christianity (Orthodox, Catholic, Protestant) each share enough common ground to be called sects or traditions (even if we massively disagree on important issues) while many other groups get labeled as cults by having divergent views such as:
- denying the trinity (e.g. Oneness Pentecostals)
- having a radically different view of heaven and hell where a limited number of human souls have eternal existence (e.g. Jehovah's Witnesses)
- or ascribing to God a physical body and making Jesus and Satan out to be brothers (e.g. Mormons)
In this definition, it is also necessary to distinguish between a full blown Cult of another religion and just a "cult" in a generic sense. For example, Heaven's Gate mixed some aspects of Christian doctrine in the formulation of their beliefs, but they did not claim to be Christian and therefore are not properly a "Christian cult", just a "cult". On the other hand, Westboro Baptist Church could easily qualify as a Christian Cult.
Using this definition, it is also possible to note that other religions are not "cults", they are other religions. For example, even if Islam borrowed from both Judaism and Christianity, today it makes no claims of belonging to those religions. Instead it claims to be a true religion in it's own right and should be identified as such. On the other hand, even very large groups (e.g. Mormons, Jehovah's Witnesses) that could easily claim independence as a unique religions choose not to do so and continue to be labeled as Christian Cults because they continue to claim affiliation but are rejected because of their beliefs. In fact some of these cults have cults of there own. There are several LDS Cults that claim to be the true LDS church but are rejected by it because of errant beliefs.
Interestingly, Christianity was originally considered to be a cult of Judaism by the Roman Empire. The earliest people to bear the name "Christian" were a sect or subset of Jews whose beliefs were rejected as being unorthodox by the Jewish religious leaders. In time, as the movement grew and the disparity between the beliefs became more pronounced as Jews rejected the more and more established Christian beliefs, Christianity came to be recognized as an independent religion.
There are of course alternate definitions. It is not uncommon to see Christians (particularly Fundamentalists) who use the term Cult to describe any and every religion apart from their own sect. There are reams of articles on the web expounding on how Islam is a cult, Protestantism is a cult, etc. Other definitions provide long lists of attributes, suggesting that you might be a cult if you match enough of them. These list usually include things like dominating leadership, strict adherence to rules, abuse of dropouts, pressure on members to conform, the existence of secret rituals, etc. While many of these are helpful things to understand about cults they are really not a good way to identify them. Everybody has a different list and these are really more of common factors than defining factors.
TL;DR: While it is possible to be used in a meaningful way, the variety of uses mean you should always check what the speakers definition is. The usage of the term will often tell you more about the beliefs of the person using it than those being described by it.
It might be worth mentioning that the scope of this particular site is Christianity and all its heresies, so heterodox teaching is just as much on topic as orthodox, meaning Christian Cults have just as much place here as Christianity itself. Generic cults that do not identify as primarily Christian or other religions maybe off topic.
See Wikipedia on Cults.
I got this from student handout notes from an Apologetics class by a seminary professor, which I think he in turn had based on The Kingdom of the Cults by Walter Martin.