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betweenness of points

A condition of one point being between two other points on a line.

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bisector of a segment

A line or segments that intersects a segment at its midpoint.

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collinear points

Occuring on the same line.

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complement of a set

The elements in the universe that are not in the set.

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coplanar points

Occuring on the same plane.

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disjoint sets

Sets that have no common elements.

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element

An object that belongs to a set. Sometimes the word "member" is used.

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empty set

A set that has no elements. It is sometimes called the null set.

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finite set

Any set whose elements can be counted.

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geometry

From the Greek for "earth measure."

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infinite set

Any set whose elements cannot be counted.

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intersection of sets

A set containing elements in common for all sets being considered.

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length of a segment

The distance between the endpoints of a segment.

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line segment

The set of two points and all points between.

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midpoint of a segment

A point that divides a segment into two equal segments.

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postulate

A statement accepted without proof.

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opposite rays

BA and BC are opposite rays if A, B, and C are collinear and B (the endpoint of both rays) is between A and C.

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proper subset

A subset that does not contain every element in another set.

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set

A collection or group of objects.

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space

The set of all possible points.

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subset

A set that contains only elements found in another set.

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union of sets

A set containing all elements of all sets being considered.

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ray

a part of a line extending from one endpoint and continuing in one direction indefinitely

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