The OED defines reformation as: "Improvement in form or quality; alteration to a better form; correction or removal of defects or errors". However, the word - always with the definite article and an initial capital 'R' - is often used to refer to a particular change or "improvement". The Reformation is the great religious movement that occurred in the 16th century, beginning with disagreement over one of the doctrines of the Roman Catholic Church and ending with the establishment of many Reformed and Protestant churches. It is, however, a matter of dispute how far the Reformation was in fact an "improvement".
See further Protestant Reformation.