What is #Hash in bitcoin from a layman's perspective? I am currently reading through the paper on bitcoin and being from a non IT background, I am unable to understand the use/function of hash
A hash is like a unique identifier, perhaps like a barcode, usually written in hexadecimal as a string of numbers an letters. Bitcoin specifically uses the SHA-256 hash algorithm in most places (it also uses RIPEMD160 in some places). You can 'hash' any data you like, and get such a unique identifier. For example the hash of a transaction is called the TXID (transaction ID). If you change any small part of the transaction then the entire hash changes. Same with blocks, the hash of a block header is used to uniquely identify that block and make sure nothing has changed. Anyone can verify that the header hashes to the same hash, to test for integrity. And it is also used to make mining difficult - a special requirement is placed on each block hash that it has to be less than some target (or that it has to start with a certain number of 0s). One of the key aspects of a hash is that there is no way to reverse it, hashing is a one way function, you cannot take a hash and find out the data it came from.