Commonly abbreviated as D, depth is a measurement of how far back a three-dimensional object is. For example, measurements of an object, such as a computer monitor, are commonly measured as (D x W x H), short for Depth by Width by Height. In the illustration, the Z-axis is the depth.
Knowing the depth 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 depth with the monitor's depth to determine if it would be too big for the desk and surrounding area.
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 haven enough depth but not fit vertically (height) or horizontally (width).