How many parts interact, and how tangled their interaction is. Complexity is not difficulty; it is the number of ways to be wrong.
Latin complexus (embraced, entwined), past participle of complecti (to encircle, comprehend), from com- (together) + plectere (to weave, braid); parts braided together closely enough that pulling one affects the rest.
Latin plectere is to weave or braid, and complexity is that braiding measured, not the effort of undoing it. A single strand pulled from a tight braid can shift every other strand, which is the whole difference between complexity and mere difficulty.