a person who knows a lot about a subject
savant - sage
A ‘savant’ is somebody with a high level of knowledge or skill, whether due to great learning or natural ability. This formal word tends to be used to refer more specifically to a person with an advanced understanding of a specialized field rather than a wide range of subjects. By extension, it can also be used in relation to a person with an intellectual disability, like autism, who may, as a result, have an exceptional skill in one thing, like mathematics.
John seems awkward and uncommunicative, but he is a mathematical savant. That’s why so many companies want to hire him.
I’m not a savant, so I can’t predict the markets with any degree of certainty, but I do have a strong gut feeling about this stock.
Karl is a political savant. He has masterminded a number of electoral victories for his clients with his uncanny ability.