Geometry Glossary - A

Absolute Value:

the distance a number is from zero on the real number line; the absolute value of x, denoted |x|, is equal to x if x  0 and |x|=-x if x < 0

Acute angle:

an angle having a measure between 0° and 90°

 

Acute triangle:

a triangle which is neither obtuse nor right; in other words, its angles are all acute

Adjacent Angles:

a pair of angles that share a vertex and a side, but no interior points

Alternate Exterior Angles:

pairs of these angles occur when two lines are intersected by a transversal; they are located on opposite sides of the transversal on the "outside" of the intersected lines

Alternate Interior Angles:

pairs of these angles occur when two lines are intersected by a transversal; they are located on opposite sides of the transversal on the "inside" of the intersected lines

Altitude: (height)

the distance between the "top" and "bottom" of a figure

Altitude of a parallelogram:

Altitude of a Prism or Cylinder:

a segment perpendicular to the parallel bases;

if the prism is right, then any lateral edge is congruent to the altitude;
if the prism is oblique, then the altitude may connect one base to the plane containing the other base

Altitude of a Pyramid or Cone:

a perpendicular segment connecting the tip of the pyramid (or cone) to the plane containing its base;

if the pyramid (cone) is oblique, the altitude may lie outside the surface;
if the pyramid (cone) is right, the altitude connects the tip to the center of the base

Altitude of a trapezoid:

a perpendicular to the two parallel sides

Altitude of a triangle:

a perpendicular from a vertex to its opposite side

Angle:

the union of two rays (or segments), which meet at an endpoint (called the vertex); angles may be classified as acute, right, or obtuse (some sources include straight angles that measure 180° and zero angles that measure 0°)

Antilogarithm:

the number corresponding to a base raised to a logarithmic power

Apothem:

in a regular polygon, it is the distance from the center to a side

Arc:

the portion of a curve (usually of a circle) between and including the two points on the curve

Area:

 the number of square units covering the interior region of a closed plane figure

Asymptote:

a line that is approached by a graph, but never touched by the graph

Axes:

the horizontal (x-axis) and vertical (y-axis) lines dividing the coordinate plane into four quadrants