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 a...

In the case of 9 th , the superscript shown is indeed called the 'ordinal indicator' as Matt Gutting has noted in his answer. The addition of...

First, these abbreviations are not onomatopoeia . In fact, your question is about writing rather than sounds . In the written number  2 nd , th...

These are ordinal suffixes . Ordinal formatter: add ordinal suffixes (-st, -nd, -rd, -th) to numbers. finzi.psych.upenn.edu