SME is the commonly used acronym for Subject Matter Expert.
Someone who has a vast knowledge on a certain topic who is often used as the fount of information from which blogs and other content are created. A subject matter expert will be known as the go-to source for any information regarding the topic at hand. For example, most business owners are subject matter experts about their business. Ask them anything related to day to day operations as well as the products and/or services they offer and you should get a very knowledgeable response.