| A set of words and terms that have specific meanings in a subject is called the terminology of that subject. Every subject has its own terminology. Each word or term of a terminology is described by known words and commonly understandable expressions. Such descriptions are called definitions. Geometry has its own terminology and definitions. Knowing the definitions helps us to understand geometry and communicate with others about geometric issues.
Point
http://mathworld.wolfram.com/Point.html
There are some terms in geometry whose meanings are very clear and simple. There are no simpler words to be used to define these terms. Such terms are called undefined terms. Point, line, and plane are the undefined terms. We consider a point as the basic element of geometry which has no length, width, or thickness (in other words, it has zero dimensions). Points are represented by dots and named by the capital letters (Figure 1.1). They are used to represent a location or a position. For example, to represent the situation of the place where two adjacent walls and the ceiling of a room meet each other (Figure 1.2), we use a point (point C in Figure 1.2).
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Figure 1.1 |
Figure 1.2 |
Line
http://mathworld.wolfram.com/Line.html
If an infinite number of points lie next to each other without any space
between them, a line is formed. In other words, a line consists of
an infinite number of points. Graphically, a line is represented by a straight
segment with arrows at the ends (Figure 1.3). A line is denoted by the
labels of its end points with “
“ above the letters. For example, the line in Figure 1.3 is represented by
 A line always extends indefinitely from the both ends. It has only one dimension, length. It can also be viewed as the path of a point that has been and will always be traveling without changing direction.
When three or more points all lie on the same line, they are called collinear. Two points will always lie on the same line.
Figure 1.3
Ray
http://www.mathopenref.com/ray.html
If a line is limited from one end, it is called a ray (Figure 1.4).
The point where the ray is limited is called the endpoint (point
C in Figure 1.4). A ray is denoted by the labels of endpoint followed by another point on the ray, with
Figure 1.4
Line Segment
http://www.homeschoolmath.net/teaching/g/angles.php
If a line is limited from the both ends it is called a line segment
or simply a segment. It is represented by the labels of its endpoints with “
“ above them. For example, the lines segments in Figure 1.5 are
 and
 . If some lines or segments
lie on a plane, they are called coplanar lines or coplanar
segments.
Figure 1.5
Plane
http://www.learner.org/channel/courses/learningmath/geometry/keyterms.html#p
When infinite number of lines lie next to each other a plane is
formed. A floor, a table top, or a wall can be considered as models of
plane. A plane is usually represented by a parallelogram (Figure 1.6) and
denoted by four capital letters or simply by one capital letter. For
example, the plane in Figure 1.6 can be identified as plane ABCD or
just plane P. Planes are extended from all the edges indefinitely.
They have width and length, but no thickness.
Figure 1.6
Angles
http://library.thinkquest.org/2647/geometry/angle/measure.htm
When two rays have a common initial point, an angle is formed. The
common point of the rays is called the vertex and rays are called the
sides of the angles. An angle is denoted by
followed by the labels used for the endpoints of the sides and the
vertex. For example, the angle in Figure 1.7 is denoted by
JKL.
Figure 1.7
A segment or a line that passes through the vertex of an angle and splits
it into congruent parts is called the bisector of the angle. In
Figure 1.8,  is the bisector
of CAD. Therefore,
CAB
 BAD.
Figure 1.8
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for Students, Parents and Teachers
Now let's do Practice
Exercise 1-1 (top).
Next Page:
Measuring Angles (top)
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