Friends,
the page https://www.blender.org/manual/modeling/modifiers/generate/mirror.html reads, in part
Mirror Object
The name of another object (usually an empty), to be used as the reference for the mirror process: its center and axes will drive the plane(s) of symmetry. You can of course animate its position/rotation to animate the mirror effect.When this page is updated, might I suggest that this paragraph be slightly revised to read
Mirror Object
Another named object (usually an empty), used as the reference for the mirror process: its center and axes will drive the plane(s) of symmetry. You can of course animate its position/rotation to animate the mirror effect.to reflect the fact that the Mirror object is an object, and not a name. In olden days, I might have made the necessary edit myself, but I've not had the time to learn how to do so under the current scheme.