When referring to a planar object, "fixed" means that the object is regarded as fixed in the plane so that it may not be picked up and flipped. As a result, mirror images are not necessarily equivalent for fixed objects.
Fixed
See also
Free, Mirror ImageExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Fixed." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/Fixed.html