In the Old Testament, the chest that contained the Tablets of the Law as given to Moses. It is now the cupboard in a synagogue in which the Torah scrolls are kept.
The original ark was built under Moses' direction when the Israelites were wandering in the desert, and was carried on poles by the priests. King David built the first Temple in Jerusalem to give a permanent home to the Ark, which was kept in the Holy of Holies.