Class 9 Maths Euclid Geometry | Euclids Definition |

**Euclid’s Definition:**

Euclid gave more than 100 definitions in the book elements. Few of them are:

- A point is that which has no part.
- A line is breathless length.
- A straight line is a line which lies evenly with the points on itself.
- A surface is that which has length and breadth only.
- A plane surface is a surface which lies evenly with the straight lines on itself.
- The extremities of a surface are lines.
- An obtuse angle is an angle greater than a right angle.
- An acute angle is an angle less than a right angle.
- A boundary is that which an extremity of anything is.
- A figure is that which is contained by any boundary or boundaries.
- A circle is a plane figure contained by one line such that all the straight lines falling upon it from one point among those lying within the figure are equal to one another; and the point is called the center of the circle.
- A diameter of the circle is any straight line drawn through the center and terminated in both directions by the circumference of the circle, and such a straight line also bisects the center.
- A semicircle is the figure contained by the diameter and the circumference cut off by it. And the center of the semicircle is the same as that of the circle.
- Parallel straight lines are straight lines which, being in the same plane and being produced indefinitely in either direction, do not meet one another in either direction.
- Equal circles are those the diameters of which are equal, or the radii of which are equal.
- An angle of a segment is that contained by a straight line and a circumference of a circle.
- A sector of a circle is the figure which, when an angle is constructed at the center of the circle, is contained by the straight lines containing the angle and the circumference cut off by them.
- A solid is that which has length, breadth, and depth.
- An extremity of a solid is a surface.
- Parallel planes are those which do not meet.
- A cube is a solid figure contained by six equal squares.

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