El Niño-Southern Oscillation is a periodic change in the atmosphere and ocean of the tropical Pacific region. It is defined in the atmosphere by the sign of the pressure difference between Tahiti and Darwin, Australia, and in the ocean by warming or cooling of surface waters of the tropical central and eastern Pacific Ocean. El Niño is the warm phase of the oscillation and La Niña is the the cold phase.

The oscillation does not have a specific period but occurs every three to eight years. Mechanisms that cause the oscillation remain a matter of research. The El Niño-Southern Oscillation is often abbreviated as ENSO and in popular usage is commonly called simply El Niño.

El Niño is Spanish for "the boy" and refers to the Christ child, because periodic warming in the Pacific near South America is usually noticed around Christmas. "La Niña," is Spanish for "the girl." Effects on weather vary with each event, but El Niño is associated with floods, droughts and other weather disturbances in many regions of the world. In the Atlantic Ocean, effects lag behind those in the Pacific by 12 to 18 months.

Developing countries dependent upon agriculture and fishing, particularly bordering the Pacific ocean, are especially affected. El Niño and La Niña are defined by sustained differences in Pacific-Ocean surface temperatures when compared with the average value. The accepted definition is a warming or cooling of at least 0.5°C (0.9°F) averaged over the east-central tropical Pacific Ocean.

When this happens for less than five months, it is classified as El Niño or La Niña conditions; if the anomaly persists for five months or longer, it is called an El Niño or La Niña "episode." Typically, this happens at irregular intervals of 2–7 years and lasts nine months to two years.

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