What does "difficulty" mean?

With Bitcoin, and other Proof of Work cryptocurrencies, the system regulates itself so that the validation of the last block of transactions takes around 10 minutes to be verified and added to the blockchain. Naturally, the more machines within a network, the faster they will validate the transactions. 'Difficulty', in this scenario, are the adjustments made by the own system in order to keep the frequency of blocks added and coins rewarded to its default (of around 10 minutes for Bitcoin, as previously mentioned). This adjustment can be lower or higher, depending on how many machines are participating in the network.