Witness

"Witnesses are entities that work for the blockchain by constructing new blocks. Each witness is approved by the shareholders and constructs and signs blocks from validated transactions. Every transaction made in the network is required to be validated by all witnesses eventually.
Consensus Mechanism

Who exactly is allows to produce a block at which time instant is defined by a consensus mechanism called Delegated Proof of Stake. In essence, the shareholders of BitShares (holders of the BTS token) can cast a vote for their preferred block producers on the blockchain. Those so called witnesses with the most votes are allowed to produce blocks."