1. triangle: A figure composed of three line segments that are not on the same line end to end is called a triangle.

2. Trilateral relationship: the sum of any two sides of a triangle is greater than the third side, and the difference between any two sides is less than the third side.

3. Height: Draw a vertical line from the vertex of the triangle to the line where the opposite side is located, and the line segment between the vertex and the vertical foot is called the height of the triangle.

4. midline: in a triangle, the line segment connecting the vertex and the midpoint of its opposite side is called the midline of the triangle.

5. Angular bisector: The bisector of the inner angle of a triangle intersects the opposite side of this angle, and the line segment between the vertex and the intersection of this angle is called the angular bisector of the triangle.

6. Coincidence: Two figures that can completely overlap are called congruences.

Congruent triangles: Two triangles that can completely coincide are called congruent triangles.

8. Corresponding vertices: The mutually coincident vertices in congruent triangles are called corresponding vertices.

9. Corresponding edges: The overlapping edges in congruent triangles are called corresponding edges.

X. Correspondence angle: The mutually coincident angles in congruent triangles are called correspondence angles.