Expected hash power required to mine a block. As more hash power is spent searching for blocks, the difficulty gets adjusted proportionally to keep the block rate and growth of the blockchain at its target rate on average.
Different coins adjust their difficulty in different ways - known as the Difficulty Adjustment Algorithm (DAA). Some adjust the difficulty over multiple weeks, some adjust it every few days, and some adjust it every block confirmation.
Frequent adjustments are valuable so that a sudden change in hash rate won't lead to super fast blocks (turbo blocks), or no blocks (death spiral).
* All terms and definitions may update as the Cryptionary improves.