The term "El Niño" has evolved over time, leading to confusion in its use. This paper reviews the meaning of the term and explores possible definitions that match historically identified El Niños after 1950. It suggests that an El Niño occurs if the 5-month running mean of sea surface temperature (SST) anomalies in the Niño 3.4 region (5°N–5°S, 120°W–170°W) exceeds 0.4°C for 6 months or more. With this definition, El Niños occur 31% of the time and La Niñas occur 23% of the time. The histogram of Niño 3.4 SST anomalies reveals a bimodal character. The definition allows the beginning, end, duration, and magnitude of each event to be quantified. Most El Niños begin in the northern spring or summer and peak from November to January in sea surface temperatures. The term "El Niño" originally referred to a warm ocean current along the coast of Peru and Ecuador around Christmas. It later became associated with the large warmings that occur every few years and change the local and regional ecology. The atmospheric component tied to El Niño is termed the "Southern Oscillation," and the phenomenon where the atmosphere and ocean collaborate is called ENSO (El Niño–Southern Oscillation). El Niño corresponds to the warm phase of ENSO, while La Niña is the cold phase. Various definitions of El Niño have been proposed, but none have achieved general acceptance. The definition proposed by Glantz includes multiple meanings of the term, but is not quantitative. Several attempts have been made to create quantitative definitions, but they have not been universally accepted. The Japan Meteorological Agency (JMA) provides an objective procedure for defining El Niño, which is consistent with the consensus of the ENSO research community. This definition involves analyzing monthly mean SSTs in 2°×2° grids, calculating monthly SST anomalies, and finding periods during which the 5-month running mean of SST anomalies exceeds +0.5°C for at least six consecutive months. The paper also discusses the recent ENSO record, showing that the Niño 3.4 region is the key region for coupled atmospheric–ocean interactions in ENSO. The standard deviations of Niño 3 and Niño 3.4 monthly anomalies for 1950–79 are 0.79°C and 0.77°C, respectively. The best match with historical judgements was achieved for the Niño 3.4 index for a threshold of 0.4°C. This definition allows for the identification of El Niño and La Niña events, with El Niños occurring 31% of the time and La Niñas 23% of the time. The paper concludes that theThe term "El Niño" has evolved over time, leading to confusion in its use. This paper reviews the meaning of the term and explores possible definitions that match historically identified El Niños after 1950. It suggests that an El Niño occurs if the 5-month running mean of sea surface temperature (SST) anomalies in the Niño 3.4 region (5°N–5°S, 120°W–170°W) exceeds 0.4°C for 6 months or more. With this definition, El Niños occur 31% of the time and La Niñas occur 23% of the time. The histogram of Niño 3.4 SST anomalies reveals a bimodal character. The definition allows the beginning, end, duration, and magnitude of each event to be quantified. Most El Niños begin in the northern spring or summer and peak from November to January in sea surface temperatures. The term "El Niño" originally referred to a warm ocean current along the coast of Peru and Ecuador around Christmas. It later became associated with the large warmings that occur every few years and change the local and regional ecology. The atmospheric component tied to El Niño is termed the "Southern Oscillation," and the phenomenon where the atmosphere and ocean collaborate is called ENSO (El Niño–Southern Oscillation). El Niño corresponds to the warm phase of ENSO, while La Niña is the cold phase. Various definitions of El Niño have been proposed, but none have achieved general acceptance. The definition proposed by Glantz includes multiple meanings of the term, but is not quantitative. Several attempts have been made to create quantitative definitions, but they have not been universally accepted. The Japan Meteorological Agency (JMA) provides an objective procedure for defining El Niño, which is consistent with the consensus of the ENSO research community. This definition involves analyzing monthly mean SSTs in 2°×2° grids, calculating monthly SST anomalies, and finding periods during which the 5-month running mean of SST anomalies exceeds +0.5°C for at least six consecutive months. The paper also discusses the recent ENSO record, showing that the Niño 3.4 region is the key region for coupled atmospheric–ocean interactions in ENSO. The standard deviations of Niño 3 and Niño 3.4 monthly anomalies for 1950–79 are 0.79°C and 0.77°C, respectively. The best match with historical judgements was achieved for the Niño 3.4 index for a threshold of 0.4°C. This definition allows for the identification of El Niño and La Niña events, with El Niños occurring 31% of the time and La Niñas 23% of the time. The paper concludes that the