Terminology
In Greek, the subscript is called ὑπογεγραμμένη (hypogegramménē, the perfect participle form of the verb ὑπογράφω, hypográphō, 'write below'). Analogously, the adscript is sometimes called προσγεγραμμένη, from the verb προσγράφω, 'to write before'.
The Greek names are grammatically feminine participle forms because in medieval Greek the name of the letter "iota", to which they implicitly refer, was sometimes construed as a feminine noun (unlike in classical and in modern Greek, where it is neuter.) The Greek terms, transcribed as ypogegrammeni and prosgegrammeni respectively, were also chosen for use in character names in the computer encoding standard Unicode.
As a phonological phenomenon, the original diphthongs denoted by 〈ᾳ, ῃ, ῳ〉 were traditionally called "improper diphthongs" in older English works. More recent works prefer the term "long diphthongs".
Read more about this topic: Iota Subscript