Chouvihar (also written Chauvihar/Chovihar) is a Jain vow of night-time restraint.

• What it means: After sunset, one does not take any food or drink until the next sunrise. This is done to protect night-time living beings (ahimsa) and to develop self-control (sanyam). Some people observe it every night, while others practice it more strictly on special days (like Paryushan). In everyday practice, many shravakas/shravikas eat before sunset and avoid eating or drinking after sunset until the next morning. There are stricter forms (Chouvihar Upvas) where no food or water is taken for the whole day, with only boiled water allowed during daylight in some traditions. Differences between Digambar and Shwetambar communities exist in the exact rules about water and what is allowed during the day.

• Attham (three-day fast) related term: In some contexts, “atham” refers to a three-day upavāsa where one fasts for three continuous days, often with no food and sometimes with boiled water allowed during the day, depending on tradition. This is a stricter form connected to Chouvihar practice in the sense of extended restraint.

If you’d like, I can summarize how Digambar and Shwetambar communities each describe these vows in their ritual manuals.