GEOMETRY VOCAB
1. Collinear – points that lie on the same line
2. Coplanar – points, lines, or rays that lie in the same plane
3. Opposite rays – rays with a common endpoint that extend in opposite
directions (form a line)
4. Acute – an angle that measures between 0 and 90 degrees
5. Obtuse – an angle that measures between 90 and 180 degrees
6. Bisector – divides an angle or line segment into two equal parts
7. Midpoint – a point that divides, or bisects, a segment into two congruent
segments
8. Supplementary – two angles whose sum is 180 degrees
9. Complementary – two angles whose sum is 90 degrees
10. Vertical angles – two angles whose sides form two pairs of opposite
rays (vertical angles are congruent)
11. Equiangular – a polygon whose angles are all congruent
12. Equilateral – a polygon whose sides are all congruent
13. Regular polygons – polygons that are both equilateral and equangular
14. Transitive property – if a=b and b=c, then a=c
15. Parallel lines – coplanar lines that never intersect (and have the same
slope)
16. Perpendicular lines – lines that intersect at a 90 degree angle (and have
slopes that are opposite reciprocals)
17. Slope-intercept form: y=mx+b (m=slope; b=y-intercept)
18. Isosceles Triangle – A triangle with at least two congruent sides (and at
least two congruent angles)
19. Scalene Triangle – a triangle with no congruent sides
20. Proportion – an equation that states that two ratios are equal
21. Pythagorean theorem – applies to right triangles, a2+b2=c2 (where a and
b are the legs, and c is the hypotenuse)
22. Transformation – an operation that moves or changes a geometric
figure in some way to produce a new figure (translation, rotation, reflection,
or dilation)
23. Reflection – a transformation that uses a line of reflection to create a
mirror image of the original figure (“flip”)
24. Rotation – a transformation in which a figure is turned about a fixed
point called the center of rotation (“turn”)
25. Translation – a transformation that moves every point of a figure the
same distance in the same direction (“slide”)
26. Surface Area – the sum of the areas of the faces of a solid
27. Circumference – distance around a circle (C=2πr)
28. Cylinder – a solid with congruent circular bases that lie in parallel
planes
29. Pyramid – a polyhedron in which the base is a polygon and the lateral
faces are triangles with a common vertex
30. Cone – a solid that has one circular base and a vertex that is not in the
same plane as the base
31. Sphere – the set of all points in space equidistant from a given point
called the center of the sphere
32. Hemisphere – half of a sphere
33. Prism – a polyhedron with two congruent faces, or bases, that lie in
parallel planes
34. Adjacent angles – two angles that share a common vertex and side, but
have no common interior points
35. Linear pair – two adjacent angles whose noncommon sides are
opposite rays
36. Concave polygon – a polygon that is not convex
37. Convex polygon – a polygon such that no line containing a side of the
polygon contains a point in the interior of the polygon
38. Area – the number of square units that covers a space or figure
39. Perimeter – the distance around (sum of the sides of) a polygon
40. Counter-example – a specific case that shows a conjecture is false
41. Conditional statement – a type of logical statement that has two parts, a
hypothesis and a conclusion (if/then)
42. Skew lines – lines that do not intersect and are not coplanar
43. Corresponding angles – two angles that are formed by two lines and a
transversal and occupy corresponding positions
44. Alternate Interior Angles – two angles that are formed by two lines and
a transversal and lie between the two lines and on opposite sides of the
transversal
45. Alternate Exterior Angles – two angles that are formed by two lines and
a transversal and lie outside the two lines and on opposite sides of the
transversal
46. Consecutive Interior Angles – two angles that are formed by two lines
and a transversal and lie between the two lines and on the same side of the
transversal
47. Concurrent – three or more lines, rays, or segments that intersect in the
same point
48. Scale – a ratio that describes how the dimensions in a scale drawing
are related to the actual dimensions of the object
49. Parallelogram – a quadrilateral with both pairs of opposite sides parallel
50. Rhombus – a parallelogram with four congruent sides
51. Diagonal of a polygon – a segment that joins two nonconsecutive
vertices of a polygon
52. Trapezoid – a quadrilateral with exactly one pair of parallel sides, called
bases (the nonparallel sides are legs)
53. Polyhedron – a 3-Dimensional shape (or solid) formed by polygons
54. Preimage – the original figure in a transformation
55. Ratio – a comparison of two numbers using division
56. Similar polygons – two polygons such that their corresponding angles
are congruent and the lengths of corresponding sides are proportional
57. Similar solids – two solids of the same type with equal ratios of
corresponding linear measures, such as heights or radii
58. Right angle – an angle with a measure of 90 degrees
59. Right triangle – a triangle with one right angle
60. Conjecture – an unproven statement that is based on observation
61. Median of a triangle – a segment from one vertex of the triangle to the
midpoint of the opposite side
62. Altitude of a triangle – the perpendicular segment from one vertex of
the triangle to the opposite side or to the line that contains the opposite
side (the height)
63. Angle bisector – a ray that divides an angle into two angles that are
congruent
64. Perpendicular bisector – a segment, ray, line, or plane that is
perpendicular to a segment at its midpoint