In this chapter, based on the synonymous mapping, we first give the definition of semantic entropy, that is, the core measure of semantic information. We name the unit of semantic information as semantic bit (sebit). Like the classic information theory, this measure can also be extended to semantic conditional/joint entropy. The mathematical properties of these measures are carefully discussed, such as the chain rule of semantic sequential entropy. In addition, we prove that all semantic entropies are not larger than their classic counterparts, and the latter measures are regarded as specific examples of the former measures.