Proof of Activity (PoA) is a cryptocurrency consensus mechanism which combines Proof of Work (PoW) and Proof of State (PoW) into a hybrid mining system.
In a PoA system, validators begin by applying a Proof of Work stage to block headers until they find a desirable hash. The block headers are then passed onto a PoS stage where stakeholders sign the headers, giving it their endorsement. When a block passes both the hash difficulty and stakeholder endorsement checks, it is considered valid.
The idea is to require less power consumption in the PoW stage by adding a PoS verification. Thus you don't need as much energy spent in PoW as a pure proof of work system like Bitcoin would.
Decred is the most notable cryptocurrency which uses the hybrid PoA mining system.
Return to main article: ELI5 Summary of cryptocurrency consensus mechanisms