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GEOMETRY VOCABULARY – UNIT 1 Together, let’s express the definition of each term by filling in the blanks.
Term
Ch 1 Section 1
POINT
LINE
COLLINEAR
PLANE
COPLANAR
LINE
SEGMENT
RAY
ENDPOINT
POSTULATE
Description
Diagram
Collinear points lie on the same line .
A plane is a flat surface
that has no thickness and
infinitely extends in all directions.
Points or lines that lie on the same
plane are called coplanar.
A line segment is a straight path that
begins at one point and ends at
another point. It has a finite length
A straight path that has a beginning
but no end is called a ray.
A point that begins or ends
a line segment or begins a ray
is called an endpoint
A statement we are asked to accept as
true is called a postulate (or axiom).
How to Name It
Use the word point
and a capital letter.
A point names a location in 2 or 3
dimensions. It has NO size.
A line is a straight path
that has no thickness and
infinitely extends in both directions.
G2A
Point P
Plot Point C so it’s collinear
with points A & Z.
Use a script cap:
Plane V,
or
three coplanar points
Plane FGH
MORE GEOMETRY VOCABULARY – UNIT 1
Term
OPPOSITE RAYS
Ch 1 Section 2
CONSTRUCTION
BETWEEN
COORDINATES
DISTANCE
/LENGTH
CONGRUENT
SEGMENTS
BISECT
MIDPOINT
SEGMENT
BISECTOR
PAGE 2
Description
Opposite rays share a common
endpoint form a straight line,
pointing in opposite directions.
Construction is the process we use
to create precise figures and
diagrams.
A point lies between two other
points if all 3 points are collinear.
The markings on a ruler used to
measure line segments are the
coordinates of the ruler.
Distance or length is the
absolute value of the difference
between two coordinates.
Line segments that are the same
length are congruent segments.
To bisect a segment is to divide it
into two congruent pieces.
A midpoint is a point that bisects
a line segment.
A segment bisector is any point,
line, line segment, ray, or plane
that bisects a line segment.
Diagram
Plot Points A & Z to create opposite rays.
How to Name It
We need an endpoint
to start to name any
ray.
MORE GEOMETRY VOCABULARY – UNIT 1
Term
Ch 1 Section 3
ANGLE
VERTEX of an ANGLE
INTERIOR/EXTERIOR
OF AN ANGLE
DEGREE
MEASURE
ACUTE ANGLE
RIGHT ANGLE
OBTUSE ANGLE
STRAIGHT ANGLE
Page 3
Description
An angle is a figure formed by two
rays with a common endpoint
The vertex of an angle is found at
the common endpoint of the two
rays forming the angle.
The interior of an angle is between
the rays forming the angle. The
exterior of an angle is outside the
rays forming the angle.
The measurement of 1/360 th of the
one rotation it takes to form a
circle is called a degree.
Angle measure reveals how close an
angle is to the 360O rotation of a
complete circle . Therefore, angles
are measured in degrees
An acute angle has a measure
between 0 and 90.
A right angle has a measure of
exactly 90 .
An obtuse angle has a measure
between 90 and 180.
A straight angle has a measure of
exactly 180 .
Hang In There, Still More …
Diagram
How to Name It
There are 4 ways to name the angle below:
 its unique vertex: Y
 by number: 2
 the vertex and point on each side: XYZ or ZYX
MORE GEOMETRY VOCABULARY – UNIT 1
Term
CONGRUENT ANGLES
ANGLE BISECTOR
Ch 1 Section 4
ADJACENT ANGLES
LINEAR PAIR
COMPLEMENTARY
ANGLES
SUPPLEMENTARY
ANGLES
VERTICAL ANGLES
Page 4
Description
Congruent angles have the same
measurement.
A ray, line or line segment that
divides an angle into two
congruent angles is called an
angle bisector.
Adjacent angles share a
common side. They lay next to
each other without any gaps.
Linear pairs are adjacent angles
that form a straight angle of
180 .
The measures of two
Diagram
How to Name It
5
Why are 4 &  5
NOT a linear pair?
complementary angles
Which pair are
supplementary?
supplementary angles
Complementary?
sum to 90 .
The measures of two
sum to 180 .
When two lines intersect , the
two angles opposite each other
are called vertical angles.
MORE GEOMETRY VOCABULARY – UNIT 1
Page 5
Ch 1 Section 5
COORDINATE PLANE
COORDINATE
Ch 3 Section 1
PARALLEL LINES
Are
Coplanar and do not
Intersect .
(Note that parallel lines have arrows in
line l || line m
the middle of them.)
PERPENDICULAR
LINES
SKEW LINES
Intersect at
90o
Are lines which do not
AND are NOT
angles.
Intersect
Coplanar
line k  line l
line k is skew
to line m
Since lines
PARALLEL
PLANES
Are planes which do not
Intersect
l and m
are parallel lines
which lie on planes
. P and R, then planes
P and R are parallel
planes.
l
P
m
R
STILL MORE GEOMETRY VOCABULARY – UNIT 1
Page 6
Ch 3 Section 3
TRANSVERSAL
A
Line
which intersects
other lines at different
Lie on the
CORRESPONDING
ANGLES
ALTERNATE
EXTERIOR
ANGLES
SAME-SIDE
INTERIOR
ANGLES
Points .
Same Side of the
transversal and the
Same Side
of the other two lines.
Lie on
ALTERNATE
INTERIOR
ANGLES
Two
1
2
3
4
&
&
&
&
5
6
7
8
Opposite sides
of the transversal and
Inside of
Lie on the
Line t is a
transversal for
lines a & b
the other two lines.
4 & 5
3 & 6
Opposite side of the
transversal and
Outside the other two lines.
Lie on the
2 & 7
1 & 8
Same Side of the
transversal and
Inside the other two lines.
4 & 6
3 & 5
WHAT!! … MORE GEOMETRY VOCABULARY – UNIT 1
Ch 4 Section 1
TRIANGLE
Any triangle with
3 acute angles.
EQUIANGULAR
TRIANGLE
Any triangle with
3 congruent angles.
RIGHT TRIANGLE
Any triangle with
1 right angle.
OBTUSE
TRIANGLE
Any triangle with
1 obtuse angle.
EQUILATERAL
TRIANGLE
Any triangle with
3 congruent sides.
ACUTE
ISOSCELES
TRIANGLE
LEGS OF AN
ISOSCELES
TRIANGLE
BASE OF AN
ISOSCELES
TRIANGLE
Page 7
Any triangle with at least 2
congruent sides (or angles).
The 2 congruent legs of an
isosceles triangle.
The base is the non- congruent
side of an isosceles triangle.
BASE ANGLES OF
The 2 angles opposite the legs
AN ISOSCELES
TRIANGLE
of an isosceles triangle.
The legs of the isosceles
triangle measures ____ units.
Mark the base of the
isosceles triangle with
one tic mark
Place double arc marks
on the base angles of
the isosceles triangle.
ALMOST THERE … MORE GEOMETRY VOCABULARY – UNIT 1
SCALENE
TRIANGLE
Ch 4 Section 2
INTERIOR ANGLE
Any triangle with
no congruent sides.
Any angle formed by two
sides of a triangle.
Any angle formed by one
side of the triangle and
EXTERIOR
ANGLE
REMOTE
INTERIOR
ANGLES
Page 8
extending another side of
the triangle.
An interior angle that is
not adjacent to the exterior
angle.
2, 3 & 5 are
interior angles.
OF COURSE … MORE GEOMETRY VOCABULARY – UNIT 1
Page 9
Ch 6 Section 1
A closed 2-D figure formed
polygon
polygon ABCDEF
by 3 or more line segments.
Any line segment
side of a
polygon
B
C
side EF
that forms a polygon.
(adjacent vertices)
Any common endpoint of two
vertex of a
polygon
A
vertex point D
interior angle D
sides of a polygon.
F
diagonal
D
Any line segment which connects
two non-adjacent vertices.
Any polygon which is BOTH
regular
polygon
equilateral and equiangular.
irregular
polygon
Any polygon which is NOT BOTH
equilateral and equiangular
The End … ? Nope
E
diagonal AC
STILL MORE GEOMETRY VOCABULARY – UNIT 1
convex
polygon
concave
polygon
Undefined
Term
Page 10
Any polygon where all diagonals
lie inside of the polygon.
Any polygon with at least
one diagonal lies outside
of the polygon.
The 3 undefined terms of geometry
are points, lines and planes because
they cannot be defined from
other figures.
Other figures are defined based
upon points, lines and planes.