A Birthday Attack is a type of attack that exploits the mathematics behind hashing functions. It relies on the probability of two different inputs producing the same hash output, known as a collision. The term comes from the birthday paradox, which demonstrates that it takes surprisingly few attempts to find matches among a set of randomly chosen items.

In the context of cryptocurrencies, where security depends heavily on hashing algorithms, an attacker can create multiple variations of a transaction or block to find a hash collision. If successful, they can manipulate data without detection, potentially allowing them to double-spend coins or alter transaction histories.

To mitigate the risk of Birthday Attacks, developers use secure hashing algorithms designed to resist collisions, making it computationally impractical to find two inputs that yield the same hash. Understanding this threat is crucial for ensuring the integrity and security of blockchain networks.