Lake El'nar was a large stretch of water on the planet Betazed.
As children, Deanna and Kestra Troi lived near Lake El'nar with their Betazoid mother, Lwaxana, and Human father, Ian Andrew.
Kestra later drowned in Lake El'nar when Deanna was still a baby.
In 2370, Deanna Troi visited a recreation of Lake El'nar inside her mother's mind while attempting to figure out why Lwaxana was so distressed. (TNG: "Dark Page")