relating to the theory or conceptual aspects of a subject, rather than its practical application and reality; determined or calculated based on ideal theory rather than on practice or experience;
e.g. In a theoretical sense, that is a good idea, but in reality, it will involve too large of an investment to be feasible.
(adjective)
skilled in theorizing, or tending to theorize;
e.g. The award winning theoretical physicist just published another book.