Something that impedes, stands in the way of, or holds up progress, either physically or figuratively
A difficulty that has to be resolved or dealt with.
A puzzling circumstance.
A set of moves required to complete a climb.
A question to be answered, schoolwork exercise.
Objection.
Difficulty in accepting or understanding or refusal to accept or understand.
Causing a problem; problematic; troublesome.
Difficult to train or guide; unruly.