Abstract: Terminology can reflect the developments of science, technology, and society. The register use of terminology has its own scientific, social, functional and cognitive reasons. As a term of probability theory and statistics, “Expectation Value” is mapped into other fields by the way of analogy and extending, or some other methods. The significant increase of the use frequency, the expansion of the semantics and the growth of the functions make “Expectation Value” develop and change from the specialized terminological fields into popular vocabulary, and become one of modern Chinese common words.

Key words: terminology, register usage, functional perspective, cognitive perspective