What is a hash function?

A hash function is a mathematical algorithm that takes an input (or 'message') and produces a fixed-size string of characters, which is typically a sequence of numbers and letters. This output is known as the hash value or hash code. Hash functions are widely used in various applications, particularly in cryptography and data integrity checks, because they provide a unique representation of the input data. Even a small change in the input will produce a significantly different hash output, which is essential for ensuring that data has not been altered.

In the context of blockchain technology, hash functions play a critical role in linking blocks securely. Each block contains a hash of the previous block, creating a chain that is immutable and resistant to tampering. Thus, the properties of the hash function ensure the integrity and security of the blockchain.

The other choices do not accurately describe a hash function. Creating backups of data and storing different versions of a blockchain involve other processes and tools, while visualizing blockchain transactions focuses on interpreting data rather than transforming it through hashing. Therefore, the definition provided in the correct choice encapsulates the fundamental role and functionality of hash functions in both data processing and blockchain technology.