The term El Niño refers to the large-scale ocean-atmosphere climate phenomenon linked to a periodic warming in sea-surface temperatures across the central and east-central equatorial Pacific.
This is approximately between the date line and 120 degrees west.
El Niño represents the warm phase of the El Niño or Southern Oscillation (ENSO) cycle, and is sometimes referred to as a Pacific warm episode.
It originally referred to an annual warming of sea-surface temperatures along the west coast of tropical South America.
NOAA's Climate Prediction Center, which is part of the National Weather Service, declares the onset of an El Niño episode when the 3-month average sea-surface temperature departure exceeds 0.5 degrees Celsius in the east-central equatorial Pacific.