How much effort or capacity a thing demands. Difficulty is relational — it exists between a task and whoever meets it.
Latin difficultas, from difficilis, dis- (not) plus facilis (easy), from facere (to do, make); a thing not easily done.
How much a task asks of whoever meets it, named as a relationship rather than a fixed property of the task alone. The same difficulty can be light for one hand and heavy for another.