Difficulty, as the word is used in cryptocurrency mining, is covered in full in the entry Mining Difficulty, which explains how it is calculated, how often it changes and how it affects miners' revenue. This page gives a short definition and explains the shorter name.
In a proof-of-work blockchain, miners compete to find a block whose hash falls below a target number. They do this by trying different values of a nonce until one works, which takes huge numbers of guesses. Difficulty expresses how small the target is: the higher the difficulty, the more guesses are needed on average. The network adjusts it automatically so that blocks keep arriving at a steady pace as the total hashrate rises or falls. Bitcoin recalculates difficulty every 2,016 blocks, roughly every two weeks, aiming for one block about every ten minutes; other networks adjust more often, sometimes every block. Rising difficulty squeezes miners' margins, while sharp falls in hashrate, for example after a ban on mining in a major country, lead to lower difficulty at the next adjustment.
Miners and data sites usually say "difficulty" for short, while reference works use the fuller "mining difficulty" to avoid confusion with other meanings of the word. Both describe the same quantity.