1. Mathematicity: It is the apparent and internal feature of mathematics and the attribute that distinguishes one thing from other things.

The two internal angles of an isosceles triangle are equal.

2. Mathematical definition: Mathematics gives an accurate and brief explanation of the essential characteristics of a thing or the connotation and extension of a concept.

A triangle with two equal sides is called an isosceles triangle.

3. Mathematical Theorems: Theorems refer to propositions proved on the basis of existing propositions, which can be other theorems or widely accepted statements, such as axioms.

For example, the judging theorem of line-plane verticality: if a straight line is perpendicular to two intersecting straight lines in a plane, it is perpendicular to this plane.