What Is the “Avalanche Effect” in Hashing, and Why Is It Important for Security?

The Avalanche effect is a desirable property of a cryptographic hash function where a tiny change in the input (like a single bit) results in a massive, unpredictable change in the output hash. This ensures that an attacker cannot systematically guess or manipulate input data to produce a desired hash output.

For Bitcoin, it guarantees that changing one transaction completely scrambles the block hash, forcing the miner to redo the PoW.