Advanced
Cryptography
Hash collision
Two different inputs that produce the same hash output.
In simple terms
A collision happens when two different inputs share a hash. For a secure hash, finding one must be computationally infeasible.
Analogy
Two different people ending up with the same fingerprint.
Going deeper
Because the output space is finite, collisions always exist — security only requires that finding one is hard. Generic attacks cost about 2n/2 operations (the birthday bound), so SHA-256 sits at ~2128 while SHA-1 fell at ~263 in the 2017 SHAttered attack.
Common mistake
Confusing "a collision exists in theory" with "a collision can be found in practice".