The literal and traditional meaning of the word "cult," from the Latin cultus, meaning care or adoration, is "a system of religious belief or ritual; or: the body of adherents to same."
Usage has narrowed informally over time to include adherence to any tight-knit minority movement centered around a given personality or idea, even in a purely secular sense. There could have been said to have been, for instance, a "Beatles cult" in the 1960s and 1970s, these being the group of followers who were more or less obsessed with the musical group The Beatles, their music, and any number of urban legends concerning their lives and activities.
Modernly, in English-speaking countries, especially in North America, critics of newer and more unusual (for the culture) religions have adopted the term in a pejorative sense to denote groups whose validity they challenge and which are generally regarded as harmful to their members. The term has fallen into disrepute among serious researchers of religion and sociology, because of the inexact and inaccurate uses which have grown popular.
In non-English languages in Europe, the cognates of the English word "cult" are neutral or common appellations to refer mainly to divisions within a single faith, a purpose to which "sect" is put in English.
In academic research, the term is used differently by psychiatrists and sociologists. Many psychiatrists have published papers based on or attempting to support anti-cult views, attempting to prove that cults use mind-control or brainwashing and are in various ways harmful to their members.
Sociologists use the term in a different sense; it is common for sociologists to reject most research by psychiatrists on the subject. Sociologists distinguish between denominations, sects and cults as follows: denominations are well-established religious groups with a low-level of tension with the surrounding society, while cults and sects are smaller groups with high tension with the surrounding society; sects have beliefs based on the mainstream religion(s) within the society, claiming however to return it to its original purity, while cults have novel and unorthodox beliefs.
Cults are furthermore divided by sociologists into audience cults, client cults and cult movements. An audience cult is a group which publishes newsletters, magazines, holds public talks, etc.; most people associated with cult do no more than subscribe to the newsletter or magazine or attend the talks. A client cult is a group where leaders have relationships with individual followers, but individual followers engage in little communal activity (e.g. Transcedental Meditation in its heyday in the United States). A cult movement is a full-blown movement where cult members engage in communal activities, e.g. religious services.
The term cult is also used by some conservative Christians to mean groups they consider heretical.
There are two main movements today opposing what they call 'cults': the Counter-Cult Movement and the Anti-Cult Movement. The Counter-Cult Movement is composed of conservative Christians who oppose groups that they consider heretical and label cults. The Anti-Cult Movement uses 'cults' to mean religious groups they believe to be dangerous and oppressive. Many academic scholars of religion have come to the defence of groups the Anti-Cult Movement has labelled 'cults'.