La Niña is also sometimes referred to as "an ENSO cold event".
El Niño was originally recognized by fisherman off the coast of South America as the appearance of unusually warm water in the Pacific ocean, occurring near the beginning of the year. El Niño means The Little Boy or Christ child in Spanish. This name was used for the tendency of the phenomenon to arrive around Christmas. La Niña means The Little Girl. La Niña is also sometimes referred to as "an ENSO cold event".  More information on naming conventions can be found in this web page on definitions of the terms ENSO, Southern Oscillation Index, El Niño and La Niña. Also interesting is the Web page Where did the name El Niño come from?