This tag should be used for questions regarding difficulty which encodes the likelihood of one hash to succeed at mining a block. Indirectly, it is a measure of the total hashing power of all miners for a given cryptocurrency.
Difficulty is a measure of the likelihood that a given hash is a valid hash for a block. It is calculated by taking the target at difficulty 1 divided by the current mining target. The difficulty is directly correlated to the total mining power of all miners on the network. The difficulty will increase as more hash rate is brought online and will decrease when hash rate decreases.