Diameter (noun)

1. The length of a straight line passing through the center of a circle and connecting two points on its circumference.
2. A measure of the size or capacity of a cylinder or pipe, typically given as the internal diameter.
3. A linear dimension, especially of a round or cylindrical object, perpendicular to its axis of symmetry.

Origin:

Late middle english: via latin from greek diametros 'measure across', from dia- 'across' + metron 'measure'. the adjective diametrical is rare before the mid 17th century.

Examples:

1. The diameter of the wheel is one meter.
2. The diameter of the pipe is six inches.
3. The diameter of the tree trunk was measured at three feet.
4. The diameter of the planet was estimated based on its distance from the sun.
5. The diameter of the hole was just large enough to let the ball through.