Geometry Mathematics Vocabulary – Based on High School
Mathematics Standards, August 2008, 0SPI
adjacent - next to
altitude - the perpendicular distance from a vertex to the opposite side of a
plane figure; the perpendicular distance from the vertex to the base of a
pyramid or cone; the perpendicular distance between the bases of a prism or
cylinder
angle of depression – the angle between a horizontal line and the line of
sight when looking down
angle of elevation - the angle between a horizontal line and the line of sight
when looking up
angle, exterior - an angle formed by a side of a polygon and an extension of
an adjacent side
angle, inscribed - an angle whose vertex is on a circle and whose sides are
chords of the circle.
angles, alternate exterior - when a transversal intersects two lines, the
angles on opposite sides of the transversal and are outside the parallel lines.
angle, alternate interior - when a transversal intersects two lines, the
angles on opposite sides of the transversal and are inside the parallel
lines
apothem - the perpendicular line segment (distance) from the center of a
regular polygon to one of its sides
arc length- the length of an arc, or portion of a circle
bisect - divide into two congruent halves
bisector - the line, ray, or segment that bisects
chord - a segment whose endpoints are on a circle
circumscribed - a polygon whose vertices are on the circle
collinear - on the same line
compass - a tool used to make circles and arcs
construction - an accurate image of a figure made using only a straightedge
and a compass
contrapositive - a conditional statement where the hypothesis and conclusion
are reversed and negated. If p, then q→ If not q, then not p
converse - a conditional statement where the hypothesis and conclusion are
reversed If p, the q→ If q, then p
co-planar - in the same plane
cosine/cos - in a right triangle, the ratio of the length of the leg adjacent to
the reference angle to the length of the hypotenuse.
counterexample - an example that shows that a conjecture is not always
true.
endpoint - either of two points marking the end of a line segment.
equidistant - equally distant.
inscribed - an angle or polygon whose vertices are part of another figure
invalid- not true.
inverse (transformation)Inverse- a conditional statement where the hypothesis and conclution are
both negated If p, then q→ if not p, then not q.
lateral (surface area) - the curved surface of a cone or cylinder.
median (triangle)- a segment from a vertex to the midpoint of the opposite
side.
midpoint - the point on a line segment that divides into two congruent
segments.
noncollinear - not on the same line.
oblique - lines that are not horizontal or vertical; a relationship between
lines and/or plane figures that is not perpendicular or parallel.
opposite - symmetrically opposed in positon or direction.
plane - a flat surface with infinite length and width but no thinkness.
postulate - a statement that is accepted as true.
proof - a logical argument that shows why a statement must be true.
prove - to verify the accuracy of (calculation or demonstration by an
independent process).
radius of a regular polygon- the radius of the inscribed circle is the apothem
of the regular polygon; the radius of the circumscribed circle is the distance
from the center of the regular polygon to a vertex.
ray- a part of a line that has one endpoint and goes on infinitely in one
direction→
rotational symmetry- a property of a figure that is mapped onto itself by a
rotation of 180 degrees or less.
scale factor - in a dilation, the ratio of the length of a segment in the
preimage to the length of the corresponding segment in the image.
secant - the inverse of the cosine function.
sector (circle) - the region bounded by two radii and the arc they intercept.
sine/sin - in a right triangle, the ratio of the leg opposite the reference angle
to the length of the hypotenuse
skew - non-coplanar lines that do not intersect
sufficient (proof) - a condition whose truth is enough to be sure a statement
is true
tangent (circle) - a line in the plane of a circle that intersects the circle in
exactly one point
tangent/tan - in a right triangle, the ratio of the length of the leg opposite
the reference angle to the length of the adjacent leg
theorem - a mathematical statement that can be shown to be true based on
postulates, definitions, or other proven theorems