A dimension whose value controls the size, orientation, or location of an element.
A locked dimension keeps the dimension value from being changed when a connected face is moved or resized. Also, a dimension must be locked if you want to use it in the Variable Table.
It is easy to identify a locked dimension:
In the graphics window, the default display color of a locked dimension is red.
In PathFinder, a locked dimension has a lock icon in front of its name.