Abbreviated: ZK Proof
A cryptographic proof where one party reveals to the other a certain value without revealing said value.
These are viewed as very important for the scaling of EVM blockchains. Vitalik Buterin stated on a number of occasions that ZK Proof is vital for the scaling of Ethereum.
Scalability is believed to come from the fact that transactions are moved off chain for verification before being settled on the Ethereum network.
One EVM that is actively looking to bring this out is Polygon. There are project teams creating open-source solutions although none are fully functioning yet.