The Ethereum consensus mechanism, known as Ethereum 2.0 or Eth2, is an upgrade to the original Ethereum blockchain that aims to improve scalability, security, and sustainability. The core feature of Ethereum 2.0 is the transition from a Proof of Work (PoW) consensus mechanism to a Proof of Stake (PoS) mechanism.

In the PoW system used by the original Ethereum and Bitcoin, miners compete to solve complex mathematical puzzles to validate transactions and create new blocks. In contrast, Ethereum 2.0's PoS mechanism relies on validators who lock up a certain amount of Ether as a stake in the network. These validators are chosen to create new blocks and validate transactions based on the amount of Ether they hold and are willing to "stake" as collateral.

This transition offers several advantages. First, it significantly reduces the energy consumption associated with mining, promoting a more environmentally friendly network. Second, it enhances scalability by allowing the network to process more transactions per second. Lastly, PoS theoretically makes the network more secure as validators are incentivized to act in the best interest of the network since their staked Ether could be slashed if they behave maliciously.

In conclusion, the Ethereum 2.0 consensus mechanism shifts from energy-intensive mining to staking, promoting efficiency, scalability, and security within the Ethereum network.