A name first given in Germany to those who adhered to the doctrine of Luther, because in 1529, they protested against a decree of the emperor Charles V. and the diet of Spires; declaring that they appealed to a general council. The same has also been given to those of the sentiments of Calvin; and is now become a common denomination for all those of the reformed churches.
See article REFORMATION; Fell's Four Letters on genuine Protestantism; Chillingworth's Religion of the Protestants; Robertson's Hist. of Charles V. vol. 2: p. 249, 250.