In cryptography, puzzle friendliness is a property of cryptographic hash functions.
Informally, a hash function is puzzle friendly if no solution exists, which is better than just making random guesses and the only way to find a solution is the brute force method.
Although the property is very general, it is of particular importance to proof-of-work, such as in Bitcoin mining.
[1] Here is the formal technical definition of the puzzle friendliness property.
The puzzle friendliness property of cryptographic hash functions is used in Bitcoin mining.