Length

Length is a measurement. The length of something is the distance between two ends of the thing. Short means a small length. Long means much length. Short and long are opposites. For two dimensional things, length is usually the longer side.

A ruler is a tool used to measure length.

Measuring

All the sides on shapes have a length. The length is between the two points of the side. You can also find the length of any two points on a shape, even if they are not on one side.

A shape can have different lengths based on how many dimensions it takes.

A line has one size measurement. This measurement is the length.
A rectangle has two measurements that show its size. These two measurements are the length and the width of the rectangle.
The length and width of a square are the same.
A box has three measurements that show its size. These three measurements are the length, the width and the height of the box.
A circle has diameter, the length across the circle. A circle also has circumference, the length around its side.

Examples

The distance from the front of the bus to the back of the bus is 30 meters. The bus is 30 meters in length.
A piece of wood is 10 meters X 10 cm X 15 cm. The piece of wood is 10 meters in length, 10 cm in breadth and 15 cm in height.

Length of time

Length can also mean an amount of time. The length is measured by looking at the time at the start, then looking at the time at the end.

You might sit down at one o'clock. If you stand up at three o'clock, you would be sitting for two hours. The length of time is two hours.