Nausea is a status effect that affects the view of a player by warping and distorting. This does not affect a player's health or hunger 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, similar to how a player enters inside a nether portal.
Like all status effects, it can be removed by drinking milk.
Aside from commands, nausea can only be obtained through eating a pufferfish.