The definition of La Niña is the cooling of the surface of the Pacific Ocean near the Earth's equator that happens at irregular intervals.
An example of La Niña is what causes unusually wet weather.
A cold-weather event, or cold episode, in contrast to El Niño, which is often referred to as a “warm event.” In the United States, La Niña causes winter temperatures that are warmer than normal in the Southeast and cooler than normal in the Northwest. The effects of both El Niño and La Niña are seen more in the winter than in the summer. La Niña impacts agricultural products and thus is watched by traders and others involved in agricultural commodities. Typically the effects last for several months to a year. La Niña is Spanish for “the little girl.” See also El Niño.