In Jainism, a day-long fast is called upavasa. If you observe no food and no water for the whole day, it is specifically called nirjala upavasa (nirjala means "without water").

Key points:

• Upavasa: fasting in general, observing a day with restricted or no eating.
• Nirjala upavasa: a strict form where you fast without any water or food for the day.
• It is considered more austere and is practiced with discipline and mindfulness.
• There can be minor differences in practice between Digambar and Shwetambar communities, but both traditions recognize nirjala upavasa as a severe form of penance.

If you’d like, I can share the traditional guidelines (rules to follow during nirjala upavasa) or provide a simple step-by-step outline for a day-long nirjala fast.