Point
A point has no dimension. It is usually represented by a small dot. A point is a primitive notion upon which other concepts may be defined. In geometry, points are zero-dimensional; i.e., they do not have volume, area, length, or any other higher-dimensional analogue. In branches of mathematics dealing with set theory, an element is sometimes referred to as a point.
Line
A line extends in one dimension. It is usually represented by a straight line with two arrowheads to indicate that the line extends without end in two directions. The notion of line or straight line was introduced by ancient mathematicians to represent straight objects with negligible width and depth. Lines are an idealization of such objects.
Plane
A plane extends in two dimensions. It is usually represented by a shape that looks like a tabletop or wall. You must imagine that the plane extends without end, even though the drawing of a plane appears to have edges. a plane is a flat, two-dimensional surface. A plane is the two dimensional analogue of a point (zero-dimensions), a line (one-dimension) and a space (three-dimensions).
Angle
An angle consists of two different rays that have the same initial point. The rays are the sides of the angle. The initial point is the vertex of the angle. Angle is also used to designate the measure of an angle or of a rotation. This measure is the ratio of the length of a circular arc by its radius. In the case of an angle (figure), the arc is centered at the vertex and delimited by the sides. In the case of a rotation, the arc is centered at the center of the rotation and delimited by any point and its image by the rotation.
We classify angles according to their angle measure.
The measure of an angle can be approximated with a protractor, using units called degrees (°).
There are four classifications of angles:
Acute - less than 90°
Obtuse - greater than 90°
Right - exactly 90°
Straight - exactly 180° (a straight line)
When talking about angles it may also be helpful to identify points on the interior/exterior of the angle.
A point is in the interior of an angle if it is between points that lie on each side of the angle.
A point is in the exterior of an angle if it is not on the angle or in its interior.
Complimentary angles are two angles whose measures add up to 90°
Supplementary angles are two angles whose measures add up to 180°
Two angles with the same angle measures are called congruent angles.
An angle bisector is a ray that divides an angle into two adjacent angles that are congruent.
(c) JWilson
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