A "Mirror Cut" is a mirror image of another cut.
A Mirror Cut
(DO508 vs DO508M)
(PZL8012 Prism Kaleidoscope and PZL8015 Western Birds)
Another Mirror Cut
(DS509 vs DS509M)
(PZL2045 The Marmalade Cat and PZL2054 Kuan-Yin on a Rockery)
Even though the cuts appear to be identical mirror images, there are subtle differences.
The die for a Mirror Cut is built from the same paper die drawing as its non-mirrored sibling.