Basic Geometry terms

Terms	Definitions
acute triangle	a triangle whose interior angles are all acute
altitude	the perpendicular distance from the base of a geometric figure to opposite vertex (or side if parallel)
complementary angles	two angles whose measures have a sum of 90 degrees
congruent	having the same size and shape
edge	the line segment where two faces of a solid figure meet
equilateral triangle	a triangle with three congruent sides
perpendicular lines	lines that intersect at one point and form right angles
perimeter	the distance around a 2-dimensiional figure
area	the number of square units needed to cover a surface
right angle	an angle with a measure of 90 degrees
straight angle	an angle whose measure is 180 degrees
obtuse angle	an angle with a measure of more than 90 degrees but less than 180 degrees
acute angles	angles that measures greater than 0 degrees and less than 90 degrees
exterior angles	angles on the outer sides of two lines cut by a transversal
face	a flat surface of a polyhedron
hypotenuse	in a right triangle, the side opposite the right angle
interior angles	angles on the inner sides of two lines that cut by a transversal
intersecting lines	two lines that cross at exactly one point
isosceles triangle	a triangle with two congruent sides
line	a straight path that extends infinitely in opposite directions
line segment	part of a line with two endpoints
line symmetry	a type of symmetry in which a line divides a figure into two congruent parts that are mirror images
obtuse triangle	a triangle with one obtuse angle
parallell lines	two lines in the same plane that never intersect
parallelogram	a quadrilateral with opposite sides parallel and congruent
perpendicular bisector	line that intersects a line segment at its midpoint and forms right angles
plane	a flat surface that extends infinitely in all directions
polygon	a closed plane figure made of line segments
prism	a polyhedron with two parallel, congruent bases; all other faces of the prism are parallelograms
pyramid	a 3-dimensional shape whose base is a polygon and whose triangular faces meet at a single point
Pythagorean Theorem	the square of the hypotenuse (c) of a right triangle is equal to the sum of the squares of the legs (a and b)
quadrilateral	a four-sided polygon
radius	a line segment with one endpoint at the center of a circle and the other endpoint on the circle
ray	part of a line that has one endpoint and extends forever in the other direction
reflection	a transformation that produces a mirror image of a geometric figure (flip)
regular polygon	a polygon in which all sides are the same length and all angles have the same measure
rhombus	a parallellogram whose four sides are congruent and whose opposite angles are congruent
supplementary angles	two angles whose sum equals 180 degrees
sphere	a three-dimensional shape with all points on the surface equidistant from the center
scalene triangle	a triangle with no congruent sides
rotation	a transformation of a figure by turning it about a fixed point called the center of rotation or axis
right triangle	a triangle with one right angle
tessellation	a pattern of congruent shapes covering a surface without gaps or overlap
transformation	a change in size, shape, of position of a geometric figure
translation	a transformation that moves a geometric figure by sliding each of the points the same distance in the same direction
vertex	a point where 2 rays meet, where two sides of a polygon meet or where the edges of a polyhedron meet; the top point of a cone or pyramid
vertical angles	a pair of opposite congruent angles formed by two intersecting lines
volume	the number of cubic units needed to fill the space occupied by a solid