An ancient language of the Indian sub-continent.
Unlike Pali (which is chiefly associated with Buddhism), Sanskrit is employed more widely - e.g. as the predominant language for most works of Hindu philosophy.
The Buddha's teachings were first transcribed in Pali, in The Pali Canon some 2000+ years ago. In the intervening years, many subsequent and seminal works in Buddhist scholarship were written in Sanskrit.
Because of this, much of Buddhism's foundational philosophy, teachings, and commentaries, are preserved in an overlapping mix of Pali and Sanskrit, not to mention the "differing opinions" of subsequent Buddhist sects and traditions.
Therefore, in Buddhism, you'll find that many concepts have both Pali and Sanskrit terms. A well-known example is the term nirvana (Sanskrit) which canonically was first known in Pali as nibanna.