Nausea is a harmful (negative) status effect in Minecraft Java Edition 1.21.
minecraft:nausea
Give Nausea II for 60 seconds (Java Edition 1.13+; duration in seconds, then amplifier where 0 = level I):
Remove the effect from the nearest player:
Nausea warps and wobbles your screen without affecting your stats, pure visual misery inflicted mainly by eating pufferfish.
Eating pufferfish inflicts Nausea along with Hunger and Poison, which is why pufferfish are brewing ingredients, not food. In older versions, entering a Nether portal caused Nausea-style distortion, but modern survival sources are essentially the fish.
There is no potion of Nausea and no benefit to having it; milk clears it like any other effect. Map makers use Nausea through commands for disorientation sequences, which is its main legitimate use.
The effect ID for Nausea is minecraft:nausea. Java Edition 1.13+ /effect commands use this namespaced form. On Java 1.12.2 and earlier the numerical ID was 9.
Run /effect give @p minecraft:nausea 60 1 to get Nausea II for 60 seconds. The first number is the duration in seconds; the second is the amplifier (0 = level I, 1 = level II). Remove it with /effect clear @p minecraft:nausea.
Nausea is a harmful (negative) status effect in Minecraft Java Edition, based on its registry category (harmful).
Eating pufferfish is the main survival source, applying Nausea for 15 seconds alongside Hunger and Poison. Commands can apply it at any level, which adventure maps use for disorientation effects.
Drink milk to clear it instantly, or wait out the duration. The screen distortion is purely visual; your stats and movement are unaffected, so you can keep playing through it if you can stand the wobble.