Mircea Eliade’s The Sacred and the Profane defines a hierophany as an event in which one sensorily experiences a manifestation of the sacred.
A hierophany can be seen (ie, a star or an angel), felt (ie, extispicy), smelled (ie, the aroma of a burnish bush or offering), heard (ie, the voice of God) or tasted (ie, imbibing a special potion) and any combination thereof, and it can be an interruption in time (ie, events in time being shown to Ebenezer Scrooge). A hierophany is a sensory experience, and thus anything in nature can be a hierophany, even the cosmos in its entirety.
Though a hierophany can also be an imago mundi or axis mundi, they do not always overlap.
The burning bush is a hierophany, but is neither an axis mundi nor an imago mundi. It was a manifestation of God, but it certainly was not a representation of the world nor was it treated as a center of the world.