A Block Formation is made up with two "blocks"--each of which contains four people--in the form of two linked squares (like the "shadow box" Square Dance emblem). From two parallel waves, have those facing out take a step forward. Then have these people do a partner trade to face back in, but remain one step out from the original wave. This has achieved the Block or Linked-Square formation. Calls are now directed to each of the two blocks of four. Dancers within each block will perform the call without interfering or attempting to work with people in the other block.
This definition is from The Handbook of Modern Square Dancing by Jay King.
It is provided here for informational and educational purposes only.