Hash

• A digital fingerprint of some binary input.
• Hashing is the transformation of a string of characters into a usually shorter fixed-length value or key that represents the original string. Hashing is used to index and retrieve items in a database because it is faster to find the item using the shorter hashed key than to find it using the original value. It is also used in many encryption algorithms.
• From this post: 

"Hash Algorithms (Functions): A cryptographic hash function is a mathematical function used in cryptography. Typical hash functions take inputs of varying length to return outputs of fixed length. A cryptographic hash function combines the message-passing capabilities of hash functions with security properties."

• Hashing algorithms are an important weapon in any cryptographer's toolbox. They are everywhere on the internet, mostly used to secure passwords, but also make up an integral part of most cryptocurrencies such as Bitcoin and Litecoin.
• The main features of a hashing algorithm are that they are a one way function – or in other words you can get the output from the input but you can’t get the input from the output – just like elliptic curve cryptography where you can’t get the private key from the public key. The other property is that the same input creates the same output.
• Most hashing algorithms, including the SHA and RIPEMD are all descended from the MD4 family. The MD4 hashing algorithm was developed by Ronald Rivest specifically to allow very easy software implementation. The MD4 algorithm and subsequent SHA algorithms use 32 bit variables with bitwise Boolean functions such as the logical AND, OR and XOR operators to work through from the  input to the output hash.
• This is the basic process behind hashing – simply convert a number into binary then perform a set of simple functions that operate through basic standard transistor and bus processes such as AND, XOR, NOT, Rotate & OR. This is part of the reason that ASIC, or application specific chips can be designed that optimise hashing. In the case of SHA-256 chips have been specifically designed to optimise the iterations throughout the steps to increase the speed of creating a hash from an input. In the case of mining this means you can calculate more hashes per second by iterating through the nonce and extra nonce parameters and have a higher probability of winning the block reward.

https://www.cryptocompare.com/coins/guides/how-does-a-hashing-algorithm-work/

https://www.cryptocompare.com/coins/guides/how-is-a-block-hash-created/