B1
someone who has a lot of skill in something or a lot of knowledge of something
Dictionary examples:
The time management expert always arrived 30 minutes late to his own seminars. (0.0)
The expert on procrastination will give his presentation... someday. (0.0)
My cat is an expert at opening doors, but terrible at closing them. (0.0)
a computer/gardening/medical expert (39.2)
He's an expert on Japanese literature. (51.9)