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.

For example, the definition of parallelogram: two groups of quadrangles with parallel opposite sides,

Theorem: A statement that is proved to be true after logical restrictions. Generally speaking, only important or interesting statements are called theorems in mathematics. Proving theorem is the central activity of mathematics.

The nature and judgment of graphs are theorems,

Nature: Understand the form of things from an objective perspective. Nature in a broad sense is the connection between one thing and other things. If one thing can change one thing, then the two things are related.

For example, the nature of parallelogram: the opposite sides are parallel, the opposite sides are equal, the diagonal lines are equally divided, and the center is symmetrical.