The measurement of the extent of an object along its greatest dimension is called its length.

Here's why:

* Greatest Dimension: This refers to the longest side of the object, regardless of its shape.

* Extent: This refers to the distance or measurement from one end of the object to the other along that longest side.

While we often use "length" to describe the measurement of a straight line, it can also be used to describe the overall size of an object in its longest direction.