So this is something of a general Sanskrit question and less of a question to do with specifically Buddhist texts.
So adhyātma, afaik, means “inside yourself” as far as its purposes in Buddhist Hybrid Sanskrit are.
In Hinduism, it refers to some kind of oversoul. There is such a thing as the Aḍhyāṭma Upaniṣad, where the usage of aḍhyāṭma seems to be more-or-less the same as the Hindu usage of adhyātma.
Where does aḍhyāṭma come from? Why these diacritics? I’ve never seen adhi- written with a ḍ or -ātma written with a ṭ. Are they different words?