Factsheet
It's an ordinal indicator:
In written languages, an ordinal indicator is a letter, or group of letters, following a numeral denoting that it is an ordinal number, rather than a cardinal number. Historically these letters were "elevated terminals", that is to say the last few letters of the full word denoting the ordinal form of the number displayed as a superscript. The exact letters used vary in different languages.
(source: Wikipedia)
Answer from Matt Gutting on english.stackexchange.com