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The term cult comes from the Latin Cultus which just means worship. Anthropologists aren't really settled on any set definition for religion or cult, but they've got a few elements that are characteristic of both. Sacred objects or books, a system of centralized beliefs about the supernatural, etc. There's also the other sense in that a cult is a derivative of a larger parent religion. Many would say for example that Mormonism is a Christian cult, as they believe in some of Christian teachings, but have rather queer beliefs that differentiate it. In this sense I'd say that Christianity as a whole is also a cult, they're a Jewish dying/rising salvific god cult. This is pretty obvious, but they believe a lot of the same things Jews do, but they have rather queer beliefs that differentiate them from the parent religion.
In the common parlance cult is a pejorative term to reference small religions. In this sense it'd be hard to say that any of the major religions are a cult because people think of it as only referring to a smaller group. I don't think this is appropriate though.