作为历史上最成功的教科书，特别是涉及的是人类所有学术中最严格的数学领域，我们有机会从欧几里德原本中学到最早的也是最纯正的严格定义：

1. Definition 1.A point is that which has no part.

2. Definition 2.A line is breadthless length.

3. Definition 3.The ends of a line are points. 

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

A definition is a passage that explains the meaning of a term (a word, phrase or other set of symbols), or a type of thing.A chief difficulty in managing definition is the need to use other terms that are already understood or whose definitions are easily obtainable.By contrast, a dictionary definition has additional details, typically including an etymology showing snapshots of the earlier meanings and the parent language.

Rules for definition:

1. A definition must set out the essential attributes of the thing defined.

2. Definitions should avoid circularity.

3. The definition must not be too wide or too narrow.

4. The definition must not be obscure.

5. A definition should not be negative where it can be positive.