You maybe thinking of its line of symmetry or point of contact
A point on a segment that is halfway between the two endpoints is called the midpoint of the segment. It is the point that divides the segment into two equal parts.
2D shape
It is called a diagonal.
A tetrahedron, assuming the four vertices are not coplanar.
Four vertices Four equal sides Four equal angles
A line between 2 vertices in Graph theory is called an edge or an arc, although arc is usually used to denote a directed edge.
Midpoint
Topology.
A line between two verticies is called a line segment.
a middle point
The line is called a diagonal if it is drawn between two nonadjacent angles.
The apex Points of a triangle are called as vertices or else the points where two line segments meet is called as Vertices.
A side (2-dimensions) or edge (3-dimensions).
Yes, if its one corner it is called Vertex, but if its corners ,its called Vertices.
A point on a segment that is halfway between the two endpoints is called the midpoint of the segment. It is the point that divides the segment into two equal parts.
2D shape
It is called a diagonal.