It is known as Solah Upvās — also called Solbhatt or Solapakhī (Shwetāmbara usage; Sanskrit: Shodasha-upavāsa). This is a very advanced 16‑day consecutive fast taken only on boiled water during daylight hours. Digambara and Shwetāmbara both recognize the 16‑day fast; names vary but the practice and discipline are the same.

For a simple overview of Jain fast types, see this short guide: