So we can say triangle. An invariant line is a line that maps to itself. Invariant quantities are usefull in Theoretical Physics because they are strongly related to the mathematical symmetries of physical systems.
When we transform a shape using translations reflections rotations enlargements or some combination of those 4 there are sometimes points on the.
Its just a point that does not move. These points are called invariant points. The side lengths of a triangle dont change when the triangle is rotated. An invariant point is usually called a stationary point of the dynamical system since for this point the motion reduces to rest.