Public domain photograph by A. Malahoff, University of Hawaii, 1980. Figure 6.8-C, U.S. Geological Survey Professional paper 1350. Fresh pillow basalt on the northeast rim of Loihi Volcano, a submarine volcano located southeast of the Island of Hawaii. Photograph coverage is 10 by 14 meters. Public domain photograph by A. Malahoff, University of Hawaii, 1980.
The definition of a maiden name is the surname or birth name a woman has before she marries and takes her husband's last name.
If you were explaining it to a child, you would say that mother's maiden name is your mother's full name before she got married, that is, her name with her original family name (or surname), the name she used when she was a girl and a young woman, before she started using the title Mrs..