A hash collision in cryptography occurs when two different inputs produce the same output hash value using a hash function. Hash functions are designed to minimize the probability of hash collisions due to their crucial role in data integrity and security practices, including in blockchains.
However, given the fixed size of the output hash and the unlimited potential inputs, collisions are theoretically possible, albeit highly improbable. In blockchain, avoiding collisions is essential because a collision could undermine the unique identification of blocks and transactions.