Real Dictionary

brain drain
depletion of the intellectual or professional resources of a country, region, etc., esp. through emigration
noun: a loss of trained professional personnel to another company, nation, etc., that offers greater opportunity
noun: the emigration of scientists, technologists, academics, etc, for better pay, equipment, or conditions
singular noun: When people talk about a brain drain, they are referring to the movement of a large number of scientists or academics away from their own country to other countries where the conditions and salaries are better.