In Britain, this is the term for an MP who is also a minister, i.e. a member of the cabinet. Specifically, it refers to ministers who are the head of their department; for instance, Estelle Morris is currently the Secretary of State for Education and Skills. The term 'Secretary of State for XYZ' is interchangeable with the term 'XYZ Secretary'; it would also be correct to say that Estelle Morris is the Education Secretary.