Three words spelled the same when they are turned upside-down are "NOON," "SIS" and "SWIMS." Numbers that are the same when turned upside down include "1881," "1961" and "91016."
Ambigram is a word coined by John Langdon to describe words that appear the same upside-down with the help of stylized calligraphy. Langdon popularized the art form in the 1970s.
The earliest known ambigrams were drawn by Peter Newell in 1893 for his books of illustrations called "Topsys & Turvys." One early ambigram used the word "chump" in a cursive form that appeared to be the same word upside-down in the June 1908 edition of "The Strand Magazine."