Definition of Abscissa:
The abscissa of a point is the distance of the point from the y-axis. It is the x-coordinate of the point in a Cartesian coordinate system. The y-axis is the vertical line on the coordinate plane, and the x-coordinate represents the horizontal distance of a point from this line.

Explanation:
In a Cartesian coordinate system, points are represented by their coordinates (x, y). The x-coordinate represents the horizontal distance of a point from the y-axis, while the y-coordinate represents the vertical distance of a point from the x-axis.

Example:
Consider a point P with coordinates (3, 4). The abscissa of this point is 3, which means it is located 3 units away from the y-axis. The ordinate of this point is 4, indicating it is located 4 units away from the x-axis.

The Y-Axis:
The y-axis is the vertical line on the coordinate plane. It is perpendicular to the x-axis and intersects it at the origin, which is the point (0, 0). The y-axis is used to represent the vertical component of a point's position.

Importance of Abscissa:
The abscissa is essential in determining the position of a point in a coordinate system. It provides information about the horizontal distance of a point from the y-axis. By knowing both the abscissa and ordinate of a point, we can precisely locate it on the coordinate plane.

Summary:
The distance of a point from the y-axis is called the abscissa. It is the x-coordinate of the point in a Cartesian coordinate system. The abscissa is crucial in determining the horizontal position of a point and plays a significant role in locating points on the coordinate plane.