An iconoclast - pronounced with the stress on the second syllable eye-KO-no-klast, IPA: /aɪ 'kɒ nə ,klæst/ - is a person who challenges and seeks to destroy cherished or traditional beliefs and practices.
The noun iconoclasm refers to the attitudes and activities of an iconoclast, and the adjective iconoclastic - pronounced with the stress on the penultimate syllable - may be used to describe these attitudes and activities.
The word 'iconoclast' is the Greek word εἰκονοκλάστης (eikonoklastes), which in turn is a compound of the words εἰκών (eikon, image or icon) and κλάω (klao, I break). The word εἰκονοκλάστης is not found in Classical Greek: it was coined in the Byzantine period to refer to a person who destroyed or advocated the destruction of icons, i.e., images of Jesus, Mary, or one of the saints, usually painted on wood in a traditional Byzantine style and cherished as objects of devotion within the Eastern Orthodox Church.
One use of iconoclasm is as a proper noun labelling a movement significant in the history of religion. During the eighth and ninth centuries there was a prolonged and bitter dispute within the Eastern Orthodox Church about the permissibility of icons as objects of religious devotion, with two protracted periods (730-787 and 815-843) during which those who venerated icons were persecuted and many icons were destroyed. Those who advocated the destruction of icons - one of the first and most prominent was the emperor Leo III (reigned 717-741) - appealed to the second commandment, which forbids idolatry - 'Thou shalt not make unto thee any graven image ...; thou shalt not bow down thyself to them, nor serve them' (Exodus 20, vv 4-5) - and maintained that the Byzantine veneration of icons was a form of idolatry. Many also believed that various natural disasters (such as a massive volcanic eruption in the Aegean Sea in 726) and the Byzantines' lack of success against the Muslim forces which at that period threatened their empire were signs of God's displeasure at this idolatrous behaviour - and, by the same token, that the military victories of the Muslims were in part due to Islam's emphatic rejection of the use of images in religious worship. The dispute was finally brought to an end in 843 by the empress Theodora (who had assumed power on behalf of her young son Michael III). The iconoclasts were condemned as heretics, and the tracts in which they had argued against the veneration of icons were ordered to be destroyed. The opponents of the Byzantine iconoclasts are usually referred to as iconophiles (i.e., lovers of icons). For more about iconoclasm in the Byzantine empire see, e.g., Judith Herrin, Byzantium: The Surprising Life of a Medieval Empire (Allen Lane, 2007), chs. 9 & 10.
Less formal outbursts of iconoclasm are recorded in other historical times. The Reformation brought many attacks on images in Britain, for example, from Protestants who taught that hagiolatry was a heresy propounded by the Roman Catholic church.
In October 1649, Milton wrote a pamphlet called Eikonoklastēs; the term was a reference to the Greek emperors "who in their zeal to the command of God, after long tradition of idolatry in the church, took courage and broke all superstitious images to pieces". This was a polemical reply to Eikōn Basilikē, a defence of Charles I, supposedly written by himself, probably in fact by his chaplain John Gauden, which had appeared ten days after his beheading in January. (The title means 'image of the king' in Greek.)