A block chain is a special sub-type of DAG in which (1) there is only one edge from each node to a predecessor, and (2) there is only one canonical path through

A block chain is a special sub-type of DAG in which (1) there is only one edge from each node to a predecessor, and (2) there is only one canonical path through the DAG at any given time and there exists an objective set of rules for selecting this path. Usually nodes that are not part of this path are eventually forgotten in a block chain.