betweenness of points

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

bisector of a segment

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

collinear points

Occuring on the same line.

complement of a set

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

coplanar points

Occuring on the same plane.

disjoint sets

Sets that have no common elements.

element

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

empty set

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

finite set

Any set whose elements can be counted.

geometry

From the Greek for "earth measure."

infinite set

Any set whose elements cannot be counted.

intersection of sets

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

length of a segment

The distance between the endpoints of a segment.

line segment

The set of two points and all points between.

midpoint of a segment

A point that divides a segment into two equal segments.

postulate

A statement accepted without proof.

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.

proper subset

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

set

A collection or group of objects.

space

The set of all possible points.

subset

A set that contains only elements found in another set.

union of sets

A set containing all elements of all sets being considered.

ray

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