My understanding of a cult, when thinking in terms of Christianity, is a denomination or branch of Christianity, that does not base its beliefs solely on the Holy Bible, but attributes equal or even more authority to the teachings of the cults founders, as in the case of J.W.'s and Mormons, and to its traditions, even if these are in conflict with the teachings of the Bible.
True Christianity, therefore, relies solely on the Bible as the source of teaching, adding nothing to it, and taking away nothing from it.