Noun
A couch for reclining at meals, extending round three sides of a table, and usually in three parts.
A dining room furnished with such a triple couch.
Source: Webster's dictionaryGrimm, "On Food and the Body," p. 356. Guests were entertained in a finely decorated dining room ( triclinium ), often with a view of the peristyle garden. Source: Internet
They concluded that the grotto is actually a nymphaeum or underground triclinium from Neronian times. Source: Internet