The problem with some words is that their meanings may change before the dictionaries can catch up with them. The terms "evangelical," "fundamentalist," and even "Christian" have all changed in terms of popular understanding. Further, dictionaries may not accurately capture the meaning of biblical or theological terms because their definitions come from popular understanding rather than the Bible.
In the second lesson in the series on the Book of Galatians I have attempted to define the term "cult" as I use it. You may wish to look at this lesson, under the sub-title: The Definition of a Cult
I'll cut and paste it here:
The Definition of a Cult
The term "cult" is certainly loaded with emotion and bias. While we may find it easy to identify particular groups as cults, no group readily uses this term of itself. It is important for us to derive a precise meaning for the term "cult." Webster defines a cult as, "A system of religious worship or ritual . devoted attachment to, or extravagant admiration for, a person, principle, etc."
Ironically, Webster chooses to use nudism as an illustration of a cult. Combining the two parts from Webster's dictionary provides the following definition of a cult: A cult is a religious group, bound together by their attachment to a person or a principle.
It is important for us to understand the difference between what is a "cult" and apostasy. A cult tends to be a rather small group, committed to a few very highly regarded principles, often led by a very "charismatic" (enthusiastic, dynamic, attractive) person. The cultist tends to view everyone outside the group as unbelievers. The cult is usually marked by a very strong, centralized authority. Apostasy is more easily defined in terms of what it is not. Apostasy denies authority, the authority of the Person of Christ and the Scriptures. Apostasy tends to shy away from anything firmly believed, other than the right to believe what you wish. While the cultist sees himself as one of the elect few, the apostate sees himself as one of the many, the majority.
Both the cult and apostasy can lead a man to a Christless eternity. The former strongly believes the wrong thing; the latter believes in little or nothing. By this definition, the Unitarian movement would be an apostate religion; the Moonies are a cult.
It is not difficult then to understand why the "sect" of the Pharisees found in Acts 15:5 should be called a cult. Their commitment was more the preservation of Judaism than anything else. In the process of seeking to preserve Judaism, the Judaizers either deliberately or unwittingly perverted the gospel of Jesus Christ from a gospel of salvation by faith to one of faith plus works. As we consider the characteristics of this "sect," we find that these same earmarks are evident in the cults of subsequent generations.
Please note the following section on the page above as well: The Characteristics of a Cult