1. A task, problem, or situation that is hard to accomplish or solve and requires significant effort, skill, or determination
Learning a new language is a difficult challenge.
Aprender uma nova língua é um desafio difícil.
2. An invitation or dare to do something that is particularly hard or demanding
He accepted the difficult challenge to climb the mountain in winter.
Ele aceitou o desafio difícil de escalar a montanha no inverno.