Quod erat demonstrandum: at the end of a proof.
See Quadrilateral.
The plane is divided into four parts by the axes. They are called first, second, third and fourth quadrant.
ax2 + bx + c = 0.
The second degree term of an equation.
A four-sided polygon: Trapezium, Rhomboid, Rhombus, Trapezoid, Rectangle, Square, Parallelogram.
Any one of the values in a frequency distribution that divides the distribution in four parts of equal frequency.
See Division.