The definition of genus plus species difference. This definition method is the most commonly used definition method in middle school mathematics, and it is based on formulas:

"Definition of adjacent genera+difference of species = defined concept"

Among them, species difference refers to the difference between the defined concept and other concepts under the same generic concept, that is, the defined concept has attributes that other concepts of its generic concept do not have.

The definition of "parallelogram" is that two groups of parallelograms with parallel opposite sides are called parallelograms.

Reveal the definition method of extension (1) inverse definition method. This is a definition method of conceptual extension, also called inductive definition method.

For example, integers and fractions are collectively called rational numbers; Sine, cosine, tangent and cotangent functions are called trigonometric functions; Ellipse, hyperbola and parabola are called conic curves; Logical sum, negation, product operations are called logical operations and so on, which are all defined in this way.

(2) Conventional definition method. There is also a special way to reveal the definition of extension, that is, the method of revealing extension adopts convention, so it is also called convention definition method. take for example

This is a concept defined in the traditional way.