An urn was a type of large vase, employed for decorative or funerary purposes.
In 2269, James T. Kirk, Carver, and Leonard McCoy hid from the Taurean women in a large urn. This backfired when rain began to fall, nearly resulting in their death by drowning. Fortunately, the all-female landing party managed to rescue them by shattering the urn with a phaser blast on setting one. (TAS: "The Lorelei Signal")