This, from Matthew Scarborough’s blog, is a pretty good explanation of a highly contentious subject:
I should start with a bit of discussion regarding the terminology here. In many contexts the Indic languages are also referred to as the Indo-Aryan languages, that is to say, the Aryan (as a term of self-designation shared by both Indic and Iranic speakers) that are attested mainly in the Indian subcontinent. For the sake of avoidance I prefer the term Indic, since the term ‘Aryan’ in many European languages carries some semantic baggage from its misappropriation fascist and genocidal regimes in the early 20th century. At the same time, Aryan is still a term of self-designation for certain ethnic groups in India, and there are non-Aryan languages from India (most notably Dravidian and Austroasiatic, but also other isolates)m and Indo-Aryan speakers are not just primarily in present-day India but also Pakistan, Bangladesh, Nepal, and more widely in diaspora, so as a term if implied by modern day borders and geography, ‘Indic’ doesn’t make much sense either. There are good arguments for both terms, and I have settled on Indic as a general neutral cover-term, at least when I am talking about these things in English.
As an aside, if you’ve been reading carefully, you might have noticed that I also prefer the term ‘Iranic languages’ rather than ‘Iranian languages’ as to imply a larger family of languages that are not constrained to the borders of present-day Iran, and for this I find the suffix -ic rather than -ian is generally helpful to make this distinction. All of this is ultimately somewhat arbitrary, but I feel the need to justify why I am calling things the way I am. (You may disagree with me, and that is fine too.)
So, I’ve waffled along so far without yet actually answering the question ‘What is Sanskrit?’. When it comes to Old Indic there is generally made a distinction between Vedic, the oldest form of the language in which the Four Vedas, Brāhmaṇa, and older Upaniṣads were composed, and the Classical language codified by the 4th c. BCE grammarian Pāṇini. Vedic represents the older tradition of a language actually spoken as a natural spoken language, whereas Sanskrit usually refers to the learned, Classical language not unlike the status of Latin in the European Middle Ages.
The general consensus of most mainstream Indo-European linguists favours the model that the Indic languages were intrusive to the Indian subcontinent via migrations of Indo-European speaking peoples from without.
Read the rest here.