Something that perplexes.
The state or quality of being perplexed; puzzled or confused.
In information theory, a measurement of how well a probability distribution or model predicts a sample.
A puzzling circumstance.
A difficulty that has to be resolved or dealt with.
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.