El Niño (/ɛl ˈniːnjoʊ/ el NEEN-yoh, Spanish:; lit. 'The Boy') is the warm phase of the El Niño–Southern Oscillation (ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (approximately between the International Date Line and 120°W), including the area off the Pacific coast of South America. The ENSO is the cycle of warm and cold sea surface temperature (SST) of the tropical central and eastern Pacific Ocean.

El Niño is accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific. El Niño phases are known to last close to four years; however, records demonstrate that the cycles have lasted between two and seven years. During the development of El Niño, rainfall develops between September–November. 

It should be noted that the cool phase of ENSO is Spanish: La Niña, lit. 'The Girl', with SSTs in the eastern Pacific below average, and air pressure high in the eastern Pacific and low in the western Pacific. The ENSO cycle, including both El Niño and La Niña, causes global changes in temperature and rainfall.
What is an "El Niño" weather pattern?
El Niño (/ɛl ˈniːnjoʊ/ el NEEN-yoh, Spanish:; lit. 'The Boy') is the warm phase of the El Niño–Southern Oscillation (ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (approximately between the International Date Line and 120°W), including the area off the Pacific coast of South America.