Euclid’s Definitions
Euclid listed some definitions.
Some of them are
 A point is that which has no part.
 A line is breadthless length.
 The ends of a line are points.
 A straight line is a line which lies evenly with the points on itself.
 A surface is that which has length and breadth only.
 The edges of a surface are lines.

A
plane surface
is a surface which lies evenly with the straight lines on itself.
We observe that in the definitions,
some things need to be explained further.
Like
 In Definition 2  “A line is breadthless length.” , what is a length? What is breadthless?
In
this question
, we try to define some terms like parallel lines, radius of circle, square etc.