You're observing a one-day upvas (upavasa) with only water consumed during daylight. Since you didn’t eat but did drink water, this is typically called a jal-upavasa (water-fast) or simply a day-long upavasa.

Notes:

• It is not nirjala upvas, because nirjala means no water at all.
• Both Digambar and Shwetambar traditions recognize upavasa as a valid form of fasting; the exact practices or rules can vary slightly between the two saṁghs, but the basic idea—fasting for a day with limited intake—is the same.

If you’d like, tell me your tradition (Digambar or Shwetambar) and the exact day you observed it, and I can describe the textually recommended guidelines for that style of upvas in Jain scriptures.