A transformation of the plane which transforms collinear points into collinear points. A projective collineation
transforms every one-dimensional form projectively, and a perspective collineation
is a collineation which leaves all lines through a point and points through a line
invariant. In an elation, the center and axis are incident;
in a geometric homology they are not. For further
discussion, see Coxeter (1969, p. 248).