Unholy Water is not defined in the rules
There is only this single mention of "unholy water" in the rules, and there is only holy water in the game described with a mechanics effect, no unholy water. Smashing a flask of holy water has this functional effect in 5e:
If the target is a fiend or undead, it takes 2d6 radiant damage.
Earlier editions had unholy water that was created by evil clerics and did harm what now would be celestials. This "unholy water" functionality has been dropped in 5e -- mechnically, holy water created by a Death Domain evil cleric is just plain old holy water.
From a play perspective this not a big issue, as most published adventures have characters fight against evil creatures, anyways, and celestials are extremely rare to show up. So not having special unholy water simplifies and streamlines the rules.
Because there is no Unholy Water with game mechanics described in 5e, this is probably just an editing oversight from the text of earlier editions. As no gp value is given, this unholy water is not the expensive flask of holy water, and is not consumed by the spell. It has no separate cost if you use a component pouch, so in practical terms it is unlikely to come up, either. So I think for practical purposes, the simplest solution here is just to ignore the "or unholy water" text, assuming it is a leftover editing glitch.
Otherwise if you feel that you need some definition of unholy water that is distinct from holy water, as that is what the rules text here says, the DM must make up what it means. Unholy water is really not a thing outside of fantasy, so there is no dictionary entry for "unholy water".
Historical background
As this answer points out , earlier editions of the game, going back all the way to 1st Edition, made it explicit what unholy water is, how it gets created and what its effects are. The 1e DMG states (emphasis added):
CREATION OF HOLY/UNHOLY WATER
Only clerics, excluding druids, are able to prepare holy water - or unholy water in the case of evil clerics [p. 114]
Holy/Unholy Water: All forms of undead, as well as creatures from the
lower planes (demons, devils, night hags, night mares, nycadaemons,
etc.) are affected by HOLY WATER. Paladins, lammasu shedu, ki-rin, and
similar creatures of good alignment (or from the upper planes) are affected
by UNHOLY WATER [p. 65]
So unholy water was the analog of holy water, created by evil clerics (i.e. those clerics that venerate an evil deity and were evil themselves).
What could Unholy Water look like?
The word "unholy" is used a couple of times in other contexts as a parallel concept to holy. If you do not want a game mechanic effect, then it would just be some water imbued or blessed with "unholiness". I think because it is an alternative to holy water, the meaning here is not just plain, "not holy" water. It must be some water that was treated in some way to be unholy.
Having no unholy water with an effect may be simpler, but from a cosmology perspective, is maybe a bit unsatisfying: why is there some divine water that affects fiends and undead, but none that affects celestials, their opposing number in the planar hierarchy?
If the DM wants to introduce unholy water as holy water created by evil clerics with a distinct effect, most straightforward would be to use the description of holy water, changing the effect to:
If the target is a celestial, it takes 2d6 necrotic damage.
