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Topic 12 Vocabulary
Lesson 12-1
line segment –a piece of line with two ends.
ray –a piece of line with one end, and the
other goes on forever.
parallel lines –two lines are parallel if they
would never cross even if they went on
forever. (In a plane, lines that never cross
and stay the same distance apart.)
intersecting lines –two lines are intersecting
if they would ever cross. (lines that pass
through the same point.)
perpendicular lines –two lines that
intersect to form right angles (90*) or square
corners.
Topic 12 Vocabulary p.2
point –a point is one little place. (an exact
location in space.)
line –a line goes on forever. (a set of points
that goes on forever in two directions.)
plane –a plane is a flat surface that goes on
forever. (an endless flat surface.)
Lesson 12-2
angle –an angle is the distance we measure
at a vertex. (Two rays that have the same
endpoint.)
vertex –a vertex is a place where two lines,
line segments, or rays intersect. (The point
where two rays meet in an angle.)
degree ( ) –a unit of measure for angles.
Topic 12 Vocabulary, p.3
protractor –an instrument used to measure
and draw angles.
acute angle –an acute angle has LESS than
90 degrees. (-an angle whose measure is
between 0 and 90 degrees.)
right angle –a right angle has EXACTLY
90 degrees.
obtuse angle –an obtuse angle has MORE
than 90 degrees. (-an angle whose measure
is between 90 and 180 degrees.)
straight angle –a straight angle has 180
degrees.
Topic 12 Vocabulary, p.4
Lesson 12-3
polygon –a polygon has straight sides joined
together. The sides are line segments. They
are joined at vertices.
regular polygon –a regular polygon is
polygon where all of the sides and all of the
angles are equal. Multiplication is an easy
way to find the perimeter of a regular
polygon.
triangle –a triangle is a 3-sided polygon.
quadrilateral –a quadrilateral is a 4-sided
polygon.
pentagon –a pentagon is a 5-sided polygon.
hexagon –a hexagon is a 6-sided polygon.
heptagon –a heptagon is a 7-sided polygon.
Topic 12 Vocabulary, p.5
octagon –an octagon is an 8-sided polygon.
nonagon –a nonagon is a 9-sided polygon.
decagon –a decagon is a 10-sided polygon.
Lesson 12-4
equilateral triangle –an equilateral triangle
is 3-sided polygon with all 3 sides equal.
isosceles triangle –An isosceles triangle is a
3-sided polygon with two of its sides equal.
scalene triangle –a scalene triangle is a
3-sided polygon with NONE of its sides
equal.
right triangle –a right triangle is a 3-sided
polygon with one right angle. The right
angle is 90 degrees.
Topic 12 Vocabulary, p.6
acute triangle –an acute triangle is a
3-sided polygon with all acute angles. An
acute angle is less than 90 degrees.
obtuse triangle – an obtuse triangle is a
3-sided polygon with one obtuse angle. An
obtuse angle is greater than 90 degrees.
Lesson 12-5
parallelogram –a parallelogram is a 4-sided
polygon with two pairs of parallel sides.
(both pairs of opposite sides are parallel)
trapezoid –a trapezoid is a 4-sided polygon
with one pair of parallel sides.
rectangle –a rectangle is a 4-sided polygon
with two pairs of parallel sides, two pairs of
equal sides, and four right angles.
Topic 12 Vocabulary, p.7
rhombus –a rhombus is 4-sided polygon
with two pairs of parallel sides and 4 equal
sides.
square –a square is a 4-sided polygon with
two pairs of parallel sides, four equal sides,
and four right angles.
Lesson 12-6
solid figure –is a figure that has three
dimensions and takes up space.
cube –a cube is a solid figure with 6 flat
surfaces called faces. All the faces are
squares.
face –a flat surface of a polyhedron (a solid
bounded by polygons).
edge –a line segment where two faces meet
in a solid figure.
Topic 12 Vocabulary, p.8
vertex –a vertex is a place where two lines,
line segments, or rays intersect. (the point
where 3 or more edges of a solid figure
meet.)
prism –a prism is a polygon that grows
straight up. A prism is a 3-dimensional
polygon with sides that stay parallel. (a solid
figure with two congruent parallel bases and
faces that are parallelograms.)
cylinder –A cylinder is a circle that grows
straight up. A cylinder is a 3-dimensional
circle with sides that stay parallel. (a solid
figure with two circular bases that are
congruent and parallel.
cone –A cone is a circle that grows up to a
point. A cone is a 3-dimensional circle with
sides that meet. (a solid figure with one
circular base, the points on the circle are
joined to one point outside the base.)
Topic 12 Vocabulary, p.9
pyramid –a pyramid is a polygon that
grows up to a point. A pyramid is a 3dimensional polygon with sides that meet.
Lesson 12-7
net –a plane figure which, when folded,
gives the original shape of a solid.
Lesson 12-9
generalization –a general statement that can
be made about something.