uninflected

listen to the pronunciation of uninflected
Английский Язык - Турецкий язык
{s} bükümsüz
(sıfat) bükümsüz
Английский Язык - Английский Язык
That does not use inflection
That has not been inflected

It seems that many such technical words (grada, minuta, introitus) were left in the uninflected state when contracted in any customary form such as we have.

not inflected; "`boy' and `swim' are uninflected English words"
{s} not inflected (Grammar)
(of the voice) not inflected; "uninflected words"; "monotonic uninflected speech
aptotic