Use 'nebulous' for subjects that are difficult to grasp or don't have a physical form. His motivations remained nebulous even after the long discussion.
Pro Tip 2/3
Lack of Detail
'Nebulous' implies something is missing specifics that can lead to misunderstanding. Her directions were nebulous, so we got lost.
Pro Tip 3/3
Feeling of Uncertainty
'Nebulous' often captures a feeling of being unsure or indecisive. The company's future plans are nebulous at this point.