Nausea is a Status Effect that affects the vision of a player by warping and distorting the view. This does not affect a player's Health or Hunger Bar in any way, but the distortion can have a rather disorienting effect on a player. When the effect is started, the distortion grows in intensity, and when it's about to end, the distortion diminishes.
Like all status effects, it can be removed by drinking Milk.
Aside from commands, nausea can only be obtained through eating Pufferfish.