Southern oscillation refers to a seesaw shift in surface air pressure at Darwin, Australia and the South Pacific Island of Tahiti. When the pressure is high at Darwin it is low at Tahiti and vice versa. El Nino, and its sister event - La Nina - are the extreme phases of the Southern Oscillation, with El Nino referring to a warming of the eastern tropical Pacific, and La Nina a cooling.
The Southern Oscillation was discovered in 1923 by Sir Gilbert Thomas Walker a British mathematician and meteorologist. The Southern Oscillation Index (SOI) measures the monthly/seasonal fluctuations in surface air pressure differences at Tahiti and Darwin (Equation = Tahiti - Darwin).