Geometry Terms: Chapter 1

Acute Angle An angle with degree measure between 0 and 90.

Adjacent Angles Two angles that lie in the same plane, have a common vertex, and a common side, but no common interior points.

Angle bisector A ray that divides an angle into to congruent angles

Between/ Betweenness of points For any two points A and B on a line, there is another point C between A and B if and only if A, B, and C are collinear and AC + CB = AB.

Collinear Points that lie on the same plane

Complementary Angles Two angles with measures that have a sum of 90.

Concave A polygon for which there is a line containing a side of the polygon that also contains a point in the interior of the polygon.

Congruent Having the same measure.

Construction A method of creating geometric figures without the benefit of measuring tools. Generally, only a pencil, straightedge and compass are used.

Convex A polygon for which there is no line that contains both a side of the polygon and a point in the interior of the polygon.

Coplanar Points that lie in the same plane.

Degree A unit of measure used in measuring angles and arcs. An arc of a circle with a measure of 1degree is 1/ 360 of the entire circle.

Exterior A point is in the exterior of an angle if it is neither on the angle nor in the interior of the angle.

Interior A point is in the interior of an angle if it does not lie on the angle itself and it lies on a segment with endpoints that are on the sides of the angle.

Line A basic undefined term of geometry. It is made up of points and has no thickness or width. In a figure, it is shown with an arrowhead at each end. Usually named by lowercase script letters or by writing the capital letters for two endpoints, with a double arrow over the pair of letters.

Line Segment A measureable part of a line that consists of two points, called endpoints, and all of the points between them.

Linear Pair A pair of adjacent angles whose non-common sides are opposite rays.

Locus The set of points that satisfy a given condition.

Midpoint The point halfway between the endpoints of a segment.

N-gon A polygon with n sides.

Obtuse Angle An angle with degree measure greater that 90 and less than 180.

Opposite rays Two rays BA and BC such that B is between A and C.

Perimeter The sum of the lengths of the sides of a polygon.

Perpendicular Lines that form 4 right angles.

Plane A basic undefined term in geometry. It is a flat surface made up of points that has no depth and extends indefinitely in all directions. In a figure, it is often represented by a shaded, slanted 4-sided figure. Usually named by a capital script letter or by three non collinear points.

Point A basic undefined term in geometry. It is a location. In a figure, it is usually represented by a dot and named by a capital letter.

Polygon A closed figure formed by a finite number of coplanar segments called sides such that the following conditions are met: The sides that have a common endpoint are noncollinear. Each side intersects exactly two other sides, but only at their endpoints, called vertices.

Precision Depends on the smallest unit available on the measurement tool.

Ray A part of a line. It has one endpoint and extends indefinitely in one direction.

Regular Polygon A convex polygon in which all of the sides are congruent and all of the angles are congruent.

Relative Error The ratio of the half-unit difference in precision to the entire measure, expressed as a percent.

Right Angle An angle with a degree measure of 90.

Segment Bisector A segment, line, or plane that intersects a segment at its midpoint.

Sides What rays that make up angles are caled.

Space A boundless 3-D set of all points.

Supplementary Angles Two angles with measures that have a sum of 180.

Undefined Terms Words, usually readily understood, that are not formally explained by means of more basic words and concepts. Includes: point, line, and plane.

Vertex A common endpoint.

Vertical Angles Two nonadjacent angles formed by two intersecting lines.