The nerve of the cells of a Voronoi diagram.
The Delaunay triangulation corresponding to a Voronoi diagram
in R^{2} is its planar dual.
For point sites in general position, the Delaunay triangulation
is a triangulation of the sites' convex hull.

A permutation with no fixed points. In other words a rearrangement of an ordered set of things so that no element is in its original position.

A design or BIBD (Balanced Incomplete Block Design) with parameters
*(v, k, lambda, r, b)* is
an incidence system (the name 'Design' comes from the original use of these
combinatorial structures in the design of industrial and agricultural
experiments) in which a set of *v* 'points' is partitioned into
*b* 'blocks' in
such a way that any two points determine *lamda* blocks.
There are *k* points in every
block, and each
point is contained in *r* different blocks. In a more general form,
a *t*-Design
(with *t > 2*) has the property that any *t* points determine
*lambda* blocks.

The prototypical example of a design is the projective space of dimension 2
over *GF(2)* aka the Fano plane, which is a design with parameters
*(v=7, k=3,
lambda=1, r=3 and b=7)* this is also an example of a
symmetric design and a
Steiner triple system

The maximum distance between any two points in a set.

A subset of the elements of a group (written additively) with the property that if
a table of all of the differences *x-y* (with *x* and *y*
in the distinguished
subset) is made, every non-identity element of the group occurs in this
table the same number of times. The quadratic residues in *GF(11)*
form a
difference set. Suppose that there is a difference set of size *k*
in a group *G*,
then twice the binomial coefficient
must be a multiple of *|G|-1*.

A Digraph or directed graph is a graph in which the edges have an explicit direction, thus the set of edges becomes a set of ordered pairs.

Also known as a skew field. A ring in which every non-zero element has a multiplicative inverse, a division ring lacks only commutativity or it would be a field.

The quaternions are a good example of a division ring.

A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

An On-line Dictionary of Combinatorics