#BlockDAG is a data structure acronym for "Directed Acyclic Graph" that is used in some cryptocurrencies such as DAGCoin and Byteball. Unlike the traditional blockchain used in cryptocurrencies like Bitcoin, where blocks are linked linearly in a chain, in a BlockDAG, blocks can have multiple parents and contain direct references to other blocks instead of just one.

This structure allows for greater scalability and transaction speed by eliminating some of the bottlenecks associated with the linear blockchain by allowing multiple transactions to be processed simultaneously in parallel. Some cryptocurrencies using BlockDAG seek to address scalability and efficiency issues that can arise with traditional blockchains.