A relatively small group of people having (esp. religious) beliefs or practices regarded by others as strange or sinister, or as exercising excessive control over members.
The Oxford English Dictionary
The rise of interest in cults in the 1970s and 1980s cemented them in the American consciousness. The Jonestown Massacre in particular—when nearly a thousand people were brainwashed into committing mass suicide via cyanide-infused Kool Aid—left “cult” with a negative connotation that it hasn’t since been able to shake.