Commonly abbreviated as W, width is a measurement of how far across a three-dimensional object is. For example, measurements of an object, like a computer monitor, are commonly given in (D x W x H), which is short for Depth by Width by Height. In the illustration, the X-axis is the width dimension.
Knowing the width of an object helps make sure that the object fits into another object. For example, if you had an enclosed computer desk, you could compare its horizontal width with the width of the monitor to determine if it would fit into the desk horizontally.
When measuring a three-dimensional object like a computer monitor, you'll want to get all measurements. In our example of a computer monitor and enclosed desk, the monitor may fit into the desk horizontally but not vertically (height) or have too much depth.