Paradox.
The Greek philosopher Zeno (circa 460 BC) is famous for several paradoxes. In one Achilles couldn't catch a tortoise because he had first to reach the point where the tortoise started. Meanwhile, the tortoise would move to another point, etc. B. Russell has discovered paradoxes of infinity (The set of all sets that do not contain themselves - does it contain itself?) There are semantic paradoxes (All Web page authors are liars) and self-referential ones.

Parity.
The word parity applies to situations where two items or their properties may be juxtaposed as being opposites (in a certain context) of each other. Integers are of either odd or even parity when they are, respectively, odd or even. The convenience is in being able to say "two numbers of different parities" without having to explicitly mention which is which.

Pole.
Pole is a singularity of an analytic function where it behaves very nearly like a rational function. If the function is bounded around a singularity the latter is called removable.

Program.
A program is a sequence of instructions that accomplish a certain task. Often instructions are written in a machine language so that a computer may perform the task required.

Projection.
Our shadow is our projection on the ground. x is the projection on the X axis and y is the projection on the Y axis, of the point (x, y).

Quadrant

When the axes are drawn in a coordinate plane, the plane is divided into 4 sections called quadrants.

Quadrilateral

A four sided figure.

Quartile

Any one of the values in a frequency distribuion that divides the distribution into four parts of equal frequency. The first quartile is the number below which 1/4 of the values are found.

Ring.
A ring is an additive commutative group in which a second operation (normally considered as multiplication) is also defined. The multiplication must be associative, i.e. a+(b+c)=(a+b)+c and the distributive law a(b + c) = ab + ac and (b + c)a = ba + ca must hold. If a ring is also a commutative multiplicative group (of course, with 0 removed) then it's called a field.