The fundamental group of transformations of an affine space. It is a subgroup of the projective group and is represented by those projective transformations that map a fixed hyperplane of the projective space into itself (cf. Projective transformation).
Affine group. A.P. Shirokov (originator), Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Affine_group&oldid=15047