Modifier Letter Small D

In UPA, it marks a very short d-like sound.

The Uralic Phonetic Alphabet (UPA) is a system for writing the sounds of Uralic languages. It was proposed in 1901 for precise speech transcription; in UPA, superscript letters mark very short sounds. So ᵈ is a phonetic mark, not a general instruction to shrink ordinary d.

Outside UPA, the shape alone does not settle the reading. In IPA, superscripts can mark a sound's release, onset, or a weak segment; the notation key and placement determine which reading applies.

It belongs to a notation system, not everyday prose

A Unicode proposal from 2002 describes UPA's early use as concentrated in scholarly work in Finland, Hungary, Estonia, Germany, Norway, Sweden, and Russia, before the notation became known more widely. It also argued that encoding these signs as distinct characters lets phonetic distinctions survive electronic publishing, sorting, and searching.

The proposal's sample index lists ᵈ in phonetic works by Lehtisalo (1956) and Rédei (1988). UPA also uses small capitals for voicing differences, subscripts for co-articulation (two sounds made together), and turned or sideways letters for reduced sounds.