The Theological Word of the Day recently was “Protestantism”:

Protestantism

A tradition in Christianity which found its self-identity as “Protestant” in the sixteenth-century Reformation. Protestantism began when the church, according to Protestants, lost the Gospel during the middle to late middle ages and reformers began to “protest” this loss. Martin Luther, often seen as the father of Protestantism, rejected the Pope’s claims to infallible authority, believed that the Gospel was being lost to a system of works-based salvation, and confessed the Bible alone was the only infallible and ultimate source of authority for the Christian. Protestantism is not a church, but a tradition which claims to have restored or reformed the Gospel, and hence, the church. Protestantism is made up of thousands of denominations (various expressions of the Protestant faith) and boasts nearly four hundred million members world-wide.

Curiously, the definition references the Pope, but not the Roman Catholic Church. The definition from Theopedia captures this element better:

Protestantism is the movement within Christianity, representing a split from the Roman Catholic Church, which occurred during the 16th century in Europe in what is called the Protestant Reformation.

Commonly considered one of the three major branches of Christianity (along with Roman Catholicism and Eastern Orthodoxy), Protestantism represents a diverse range of theological and social perspectives, denominations and related organizations.

And the division is even more explicitly defined in the article on the Protestant Reformation:

The theology of the Reformers departed from the Roman Catholic Church primarily on the basis of three great principles:

• Sole authority of Scripture,
• Justification by faith alone, and
• Priesthood of the believer.