Those who refused to accede to the demands of the four "Acts of Uniformity" passed in England from the reign of Edward VI to that of Charles II.
By these it was required of all ministers that they accept certain conditions as to the conduct of public worship, especially the use of the Book of Common Prayer with certain prescribed ceremonies.
Those who would not conform set up for themselves separate conventicles, and became the foundation of English Protestant sectarianism.
In later usage the term Nonconformist includes every denomination outside the Church of England, Catholicism alone excepted.
New Catholic Dictionary