A round shield is a shield that is circular. At it does not possess a gap in the centre to allow for a handle to be gripped (as in a centre boss shield) it needs to have at least one strap for the forearm and a could have another that is gripped.
Less manuevourable than a centre boss shield of the same size, it also allows the transfer of energy directly to the forearm through the shield. To combat this, padding is often added behind the shield to reduce this effect.
The targe is a historical example of this type of shield.