Subject matter expert (frequently used in business, originally heard at AT&T). Original meaning was the person who was knowledgably about the subject, has now become a somewhat pejorative term for someone who just knows the facts but has no power to make a decision. (Sometimes confused with/pronounced like shmo).
(Original use:) Check with SME to find out how to get the service provisioned. (Current use:) He will not be able to get anything changed, he is just the SME.