What does "plain water" mean?

Tap water, either hot or cold, served in restaurants.

Plain water

Common synonyms include: base water, drinking water, neutral solvent, neutral water, plain h2o, still water.

noun

noun ·Rare ·Advanced level

Noun

1
Tap water, either hot or cold, served in restaurants. Singapore, uncountable
"Politely I asked for a glass of plain water."
2
Drinking water Malaysia, uncountable

"Politely I asked for a glass of plain water."