Difficulty can be defined as following:
In the current cryptocurrency network, a certain amount of hashrate is required to mine a block. The required amount of hashrate determines difficulty of mining.
Example: Suppose the current bitcoin difficulty is 3,511,060,552,899 (3.51T). In theory, miners need to provide 3.51T hashrate to mine a block, and get 12.5 bitcoin Coinbase reward (April 2018 bonus amount) .