Term signifying in classical Latin the political party or philosophical school which one followed, applied in the Epistles to divisions within the Christian communities, and not frequently used until the rise of Protestantism.
Now it is a general term signifying any organized body of dissenters from some established or older form of faith and is applied to all other denominations in countries wbere State churches exist.
It is used by the Catholic Church to designate any band of Christians who refuse to accept the doctrine and authority of the Church and constitute a religious party under human unauthorized leadership.
The recognition by the Church of all the sects which sprang up in the course of her history would have been fatal to her organization, for the dissenters generally split up into innumerable parties all holding different doctrines.
There is disapproval of the term and disagreement concerning the meaning of it among the Protestant bodies, many of which prefer the designation "churches."
New Catholic Dictionary