Euclid listed 23 definitions in book 1 of the 'elements'. We list a few of them:

1) A point is that which has no part

2) A line is a breadth less length

3) The ends of a line are points

4) A straight line is a line which lies evenly with the points on itself.

5) A surface is that which has length and breadth only.

6) The edges of a surface are lines

7) A plane surface is surface which lies evenly with straight lines on its self.

Starting with these definitions, Euclid assumed certain assumptions, known as axioms and postulates.