A fancy way to say commutative (honoring N. H. Abel). In an abelian
group , * xy = yx * for any arbitrary choice
of *x* and *y*.

The ideal generated by a set in a vector space.

Over any finite field *F* one can
construct a two-dimensional affine geometry. If the field is of size
*n*, the affine plane consists of a set *P* of points which
are ordered pairs of elements of the field (thus there are *n*^{2}
points), and a set *L* of lines which are themselves sets of points.
The points on a given line consist of all pairs *(x,y)* which are
solutions of a linear equation (* y=mx+b *, or *x=k*, with
*m,b* and * k * in *F*). The lines can be further
divided up into "parallel classes" based on the value of *m* in
their defining equation, (the lines of the form *x=k* are said
to be in the "infinite slope" parallel class).

By adding an additional "point at infinity" for each of the parallel classes and an additional "line at infinity" which connects the points at infinity, one can construct a projective plane from an affine plane.

A digraph *D* is an arborescence if
there is a distinguished node *x* (called the root) and for every
node *y* different from *x* there is exactly one elementary
path from *x* to *y*.

The partition of space into cells formed by overlaying a collection of surfaces (often hyperplanes).

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