###
points that lie on the same line

collinear

###
points that do not lie on the same line

noncollinear

###
a mathematical statement that can be proved

theorem

###
an unproved assumption

postulate

###
two collinear rays that have a common endpoint and extend in different directions

opposite rays

###
a pair of angles such that the rays forming the sides of one and the rays forming the sides of the other are opposite rays

vertical angles

###
a perpendicular segment from a vertex of a triangle to the opposite side

altitude

###
a line introduced into a diagram for the purpose of clarifying a proof

auxiliary line

###
a polygon in which each interior angle has a measure less than 180

convex polygon

###
lying in the same plane

coplanar

###
the length of the shortest path between two objects

distance

###
the first and fourth terms of a proportion

extremes

###
the point of intersection of a line and a plane

foot

###
the second and third terms of a proportion

means

###
the point of concurrency of the altitudes of a triangle

orthocenter

###
the point of concurrency of the perpendicular bisectors of the triangle's sides

circumcenter

###
the point of concurrency of the medians of a triangle

centroid

###
an equation stating that two or more ratios are equal

proportion

###
a quotient of two numbers

ratio

###
a polygon that is both equilateral and equiangular

regular polygon

###
two lines that are not coplanar

skew lines

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