Difficulty of finding two different inputs that hash to the same value
Collision resistance is a security property of cryptographic hash functions. A hash function $H$ is said to be collision resistant if it is computationally infeasible to find two input strings $s$ and $s'$ such that $s' \ne s$ but $H(s') = H(s)$.
Collision resistance is one of the strongest security properties usually demanded of hash functions: a collision resistant hash function is automatically also first and second preimage resistant.
Collision resistance is also called strong-collision.