From the Greek for "image breaking", it was opposition to the use of Christian religious images in churches
At the Council of Hieria in 754 CE, the Church endorsed an 
iconoclast ... Iconoclasm, Greek for "image-breaking," is the deliberate 
destruction within a ... the Byzantine Empire when the use of religious images or 
icons was opposed by... [DOC] [TLE] Iconoclasm | Catholic Answers [PAR] Heresy opposed to the use of religious images.