Short for Proof of Stake. This is a consensus mechanism for securing and processing transactions on a blockchain and is an alternative to Proof of Work. It requires validators to stake some crypto currency to verify transactions on the blockchain and create blocks. They are incentivised to be honest by receiving a % of all transaction fees and they dis-incentivised from being dishonest by slashing, which is where they lose some cryptocurrency of they do not process transactions correctly. It requires less computational power than Proof of Work, which is how Bitcoin runs. Ethereum expects to reduce power consumption by more than 99% when it moves from PoW to Proof of Stake.