Proof of stake

Proof of stake (PoS) protocols are a class of consensus mechanisms for blockchains that work by selecting validators in proportion to their stake in the associated cryptocurrency.
Description.
For a blockchain transaction to be recognized, it must be appended to the blockchain. Validators carry out this appending; in most protocols, they receive a reward for doing so. For the blockchain to remain secure, it must have a mechanism to prevent a malicious user or group from taking over a majority of validation. PoS accomplishes this by requiring that validators have some quantity of blockchain tokens, requiring potential attackers to acquire a large fraction of the tokens on the blockchain to mount an attack.
Comparison to other consensus mechanisms.
Proof of work (PoW), another commonly used consensus mechanism, uses computational power as its scarce resource, requiring a potential attacker to acquire a large fraction of the computational power of the validator network. This incentivizes consuming huge quantities of energy. PoS is comparatively more energy-efficient.
Attacks.
PoS protocols can suffer from the "nothing-at-stake problem", where validator nodes validate multiple conflicting copies of the blockchain because there is minimal cost to doing so, and a smaller chance of losing out on rewards by validating a block on the wrong chain. If this persists, it can allow double-spending. This can be mitigated through penalizing validators who validate conflicting chains or by structuring the rewards so that there is no economic incentive to create conflicts.
Variants.
The exact definition of "stake" varies from implementation to implementation. For instance, some cryptocurrencies use the concept of "coin age", the product of the number of tokens with the amount of time that a single user has held them, rather than merely the number of tokens, to define a validator's stake.
Delegated proof of stake.
Delegated proof of stake (DPoS) systems separate the roles of the stake-holders and validators, by allowing stake-holders to delegate the actual validation role.
Implementations.
, PoS cryptocurrencies were not as widely used as proof-of-work cryptocurrencies. The first functioning implementation of a proof-of-stake cryptocurrency was Peercoin. Other cryptocurrencies, such as Blackcoin, Nxt, Cardano, and Algorand followed. Additionally there have been sporadic proposals for Ethereum to switch from a PoW to PoS mechanism.