Definition originally refers to a clear description of the value of things. Modern definition: an accurate and brief explanation of the essential characteristics of a thing or the connotation and extension of a concept; Or describe or standardize the meaning of a word or concept by enumerating the basic attributes of events or objects; A defined transaction or object is called a defined project and its definition is called a defined project.

A brief description of the essential characteristics of a thing or the connotation and extension of a concept. It is equivalent to the assignment of unknowns in mathematics. For example, "Let an unknown be a known letter X to simplify the calculation", and give a certain meaning or image to the named words, which is beneficial to the identification and recognition in communication.