Ethash is a proof-of-work algorithm that is used by Ethereum and other cryptocurrencies. It is similar to the Bitcoin hashing algorithm, but with a few key differences.
The main purpose of Ethash is to deter denial-of-service attacks and other forms of abuse on the Ethereum network. It does this by requiring miners to use a large amount of memory when they are mining new blocks.
This makes it very expensive for an attacker to mount a successful attack.
Ethash is also designed to be ASIC-resistant. This means that it is not possible for specialized mining hardware to be developed that would give miners a significant advantage over those using regular CPUs or GPUs.
The Ethash algorithm is also used by other cryptocurrencies, including Monero and Zcash.