Given a point l and a point P. If P is an element of l,
then we say that P lies on l, or
P is incident on l, or
l passes through P.

Now we can try to find answers to the following questions on the basis of our experience.

1. Given a point P, is there a line that passes through P? how many such lines are there?
2. Given two distinct points A and B, is there a line that passes through both a and B? How many such lines are there?
3. Given a line l, is there a point that lies on it? How many such points are there?
4. Given two distinct lines l and m, is there a point that lies on both l and m? how many such points are there?

Based on the results of the above questions we can arrive at the following conclusions. These conclusions have to be taken as axioms.

Incidence Axiom 1: A line contains infinitely many points.

Incidence Axiom 2: Through a given point, there pass infinitely many lines.

Incidence Axiom 3: Given two distinct points A and B, there is one and only one line that contains both the points.

According to the third axiom, any two distinct points of the plane determine a line uniquely and completely.

Definition: Three or more than three points are said to be collinear, if there is a line which contains them all.

Definition: Three or more than three lines are said to be concurrent if there is a point which lies them all.