A zoo was a place where animals were kept in captivity to observe and study their characteristics.
In 2269 the crew of the USS Ariel was captured by the Lactrans on a survey mission. They were placed in a Lactran zoo until they were freed, about six weeks later, by the crew of USS Enterprise. (TAS: "The Eye of the Beholder")
During a meeting between high ranking members of the Federation Alliance in 2374, Romulan Senator Letant made a derogatory remark against Martok, telling an aide "Klingons can be quite entertaining, don't you agree? Every Romulan zoo should have a pair!" (DS9: "Tears of the Prophets")