A cenote is a partly water-filled, wall-sided doline. It is formed by the collapse of a cave which is (today, not necessarily at the time of the collapse) filled with water.
This sort of doline is very common in Yucatan, México, where a large cave system with many entrances is filled with water. There are over 3,000 cenotes, with only 1,400 actually studied and registered. The cave system inside the Yucatan penisula was formed during the Ice Ages, when the level of the sea was 100m lower than today. The water drained to this level and the caves were slightly above sea level. When the glaciers melted and the sea level rose, the caves were filled with water. There is sweet water from the rains inside the country, but also sea water flowing in from the sea. Some cenotes contain two layers of water, the heavier sea water on the floor, the sweet water above and a visible border surface in the middle.
Cenote is hard for English speakers to pronounce, the right pronunciation is like say-NOH-tay. The term is based on the Mayan word dzonot or ts'onot (sacred well). It was adopted by the Spanish who had no word for such circular lakes, as they are not common in Spain. That is why cenote is pronounced the Spanish way. And the term cenote just means well, as the Spanish ignored the "sacred" part.
This places were very important to the ancient Mayas, as the karst plain of Yucatan is completely waterless, and those collapses were the only possibilities to reach the ground water. Some of them were sacred places and used for offerings.