The state of being wide.
"The width of the canyon was so vast that we could barely see the other side from our viewpoint."
In plain English: Width is how wide something is from side to side.
"The width of the doorway was just enough for the sofa to fit through."
The word "width" comes from combining the old root for "wide" with a suffix meaning abstract quality. It entered common use in the late 1500s to replace older forms like "wideness."