A Smarandache Geometry (1969) is a geometric space (i.e., one with points, lines) such that some "axiom" is false in at least two different ways, or is false and also sometimes true. Such axiom is said to be Smarandachely denied (or S-denied for short). In Smarandache geometry, the intent is to study non-uniformity, so we require it in a very general way.

A manifold that supports a such geometry is called Smarandache manifold (or s-manifold for short). As a special case, in this book Dr. Howard Iseri studies the s-manifold formed by any collection of (equilateral) triangular disks joined together such that each edge is the identification of one edge each from two distinct disks and each vertex is the identification of one vertex each of five, six, or seven distinct disks.

Thus, as a particular case, Euclidean, Lobacevsky-Bolyai-Gauss, and Riemann geometries may be united altogether, in the same space, by certain Smarandache geometries. These last geometries can be partially Euclidean and partially Non-Euclidean.