Euclidean Definitions

1. A point

That which has no part

2. A line

A breadthless length

3. Points

The extremities of a line

4. A straight line

A line which lies evenly with the points on itself

5. A surface

That which has length and breadth only

6. Lines

The extremities of a surface

7. A plane surface

A surface which lies evenly with the straight lines on itself

8. A plane angle

The inclination to one another of two lines in a plane which meet one another and do not lie in a straight line

9. Rectilineal

An angle where the lines containing the angle are straight

10. Right

An angle when a straight line set up on another straight line makes the adjacent angles equal to one another

10. (cont) Perpendicular

The relation between two straight lines in which one is set up on another, such that the adjacent angles equal to one another (right angle)

11. Obtuse

An angle greater than a right angle

12. Acute

An angle less than a right angle

13. Boundary

that which is an extremity of anything

14. Figure

That which is contained by any boundary or boundaries

15. Circle

A plane figure contained by one line such that all straight lines falling upon it from one point among those lying within the figure are equal to one another

16. Center (of a circle)

the point around which any line drawn to the boundary of a figure is equidistant from all points on the boundary

17. Diameter

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 circle

18. semicircle

the figure contained by the diameter and the circumference cut off by it, the center of this is the same as that of the circle it is placed within

19. Rectilineal Figures

figures which are contained by straight lines

19. (cont) trilateral

figures contained by three straight lines

19. (cont) quadrilateral

figures contained by four straight lines

19. (cont) multilateral

figures contained by more than four straight lines

20. Equilateral Triangle

of trilateral figures, that which has its three sides equal

20. (cont) isosceles triangle

of trilateral figures, that which has two of its sides alone equal

20. (cont) Scalene Triangle

of trilateral figures, that which has its three sides unequal

21. right angled triangle

of trilateral figures, that which has a right angle

21. (cont) obtuse-angled triangle

of trilateral figures, that which has an obtuse angle

21. (cont) acute-angled triangle

of trilateral figures, that which has its three angles acute

22. square

of quadrilateral figures, that which is both equilateral and right-angled

22. (cont) oblong

of quadrilateral figures, that which is right-angled but not equilateral

22. (cont) rhombus

of quadrilateral figures, that which is equilateral but not right-angled

22. (cont) rhomboid

of quadrilateral figures, that which has its opposite sides and angles equal to one another but is neither equilateral nor right-angled

22. (cont) trapezia

of quadrilateral figures, all quadrilaterals not fitting into the classifications of squares, oblongs, rhombi, or rhomboids

23. Parallel lines

straight lines which, being in the same plane and being produced indefinitely in both directions, do not meet one another in either direction