A hash is an alphanumeric string and end product of a cryptographic hash function (CHF). The size of a hash is different depending on what CHF is used, but all hashes from a given CHF have the same size. A hash’s size is represented in storage bits (computer storage bits).
Bitcoin uses a CHF called SHA-256 (Secure Hash Algorithm) that generates a 256 bit hash. Each digit in the 256 hash represents 4 bits, so the length of the hash is an alphanumeric string of 64 characters.
A hash consists of the block header and a nonce.