For World History it means - Byzantines who opposed the use of icons in worship.
1. Literally, an iconoclast is someone who smashes images (often in places of worship).
2. In a broader sense, it is someone with no respect for authority or tradition.
1. a breaker or destroyer of images, esp. those set up for religious veneration. 2. a person who attacks cherished beliefs, traditional institutions, etc., as being based on error or superstition