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My Book of Geometric
Terms
Basic Geometric Vocabulary
EXAMPLE
VOCABULARY WORD
DEFINITION
point
a position in space
line
a straight figure that
goes on forever in
both directions
line segment
a section of a line that
has endpoints
ray
a line with one
endpoint that goes on
forever in one
direction
parallel
lines, sides, or planes
that will never
intersect or cross
perpendicular
lines, sides, or planes
that create a right
angle or 90 °
intersecting
lines, sides, or planes
that cross
side
one of the lines that
joins the
corners/vertices of a
shape
side
opposite sides
vertex
a corner formed by
two line segments,
rays, etc.
opposite sides
sides that are across
from each other
adjacent sides
sides that are next to
each other
Angles Vocabulary
EXAMPLE
VOCABULARY WORD
DEFINITION
obtuse angle
an angle whose
measure is greater
than 90°
acute angle
an angle whose
measure is less than
90°
right angle
an angle whose
measure is exactly 90°
straight angle
an angle whose
measure is exactly
180°
reflex angle
an angle whose
measure is between
180° and 360°
Polygons Vocabulary
EXAMPLE
VOCABULARY WORD
DEFINITION
polygon
a closed 2D (flat)
shape with:
 at least 3 sides
 no curved sides
 only 1 interior
regular polygon
a polygon with all equal
sides and angles
irregular polygon
a polygon whose sides
and angles are not all
equal
concave polygon
convex polygon
a polygon who has one
or more angles larger
than 180° ; a polygon
who has one side that
looks “pushed in”
a polygon whose angles
are all less than 180°
Polygons Vocabulary
EXAMPLE
VOCABULARY WORD
triangle
DEFINITION
a three-sided polygon
whose angles add up to
180°
quadrilateral
a four-sided polygon
whose angles add up to
360°
pentagon
a five-sided polygon
whose angles add up to
540°
hexagon
heptagon
a six-sided polygon
whose angles add up to
720°
a seven-sided polygon
whose angles add up to
900°
octagon
an eight-sided polygon
whose angles add up to
1080°
nonagon
a nine-sided polygon
whose angles add up to
1260°
decagon
a ten-sided polygon
whose angles add up to
1440°
Types of Triangles Vocabulary
EXAMPLE
VOCABULARY WORD
DEFINITION
equilateral triangle
a triangle with all equal
sides and equal angles
scalene triangle
a triangle with no equal
sides or equal angles
isosceles triangle
a triangle with two
equal sides and two
equal angles
obtuse triangle
a triangle with one
obtuse angle
acute triangle
a triangle with all
acute angles
right triangle
a triangle with one 90°
angle
Types of Quadrilaterals
EXAMPLE
VOCABULARY WORD
DEFINITION
parallelogram
a quadrilateral with 2
sets of opposite,
parallel sides
rectangle
a quadrilateral AND
parallelogram with
four 90° angles
rhombus
a quadrilateral AND
parallelogram with
four equal sides
square
a quadrilateral,
parallelogram,
rectangle and rhombus
that has four equal
sides and four 90°
angles
kite
a quadrilateral that
has two sets of equal
adjacent sides
trapezoid
a quadrilateral with
only one set of
parallel, opposite sides
Quadrilateral Hierarchy
Quadrilateral Facts:
 Every square is a rectangle, but not every rectangle is a square
o Not every rectangle has four equal sides
 Every square is a rhombus, but not every rhombus is a square
o Not every rhombus has 90° angles
 Every square is a parallelogram, but not every parallelogram is a square
o Not every parallelogram has 90° angles and/or four equal sides
 Every rectangle is a parallelogram, but not every parallelogram is a rectangle.
o Not every parallelogram has 90° angles
 Every rhombus is a parallelogram, but not every parallelogram is a rhombus.
o Not every parallelogram has four equal sides