Harmonic Homology

A Perspective Collineation with center $O$ and axis $o$ not incident is called a Homology. A Homology is said to be harmonic if the points $A$ and $A'$ on a line through $O$ are harmonic conjugates with respect to $O$ and $o\cdot a$. Every Perspective Collineation of period two is a harmonic homology.

See also Homology (Geometry), Perspective Collineation

References

Coxeter, H. S. M. Introduction to Geometry, 2nd ed. New York: Wiley, p. 248, 1969.