A Well is a ter'angreal that is able to store the One Power so that a channeler can use it as a separate source.[1]
Conceivably, an Aes Sedai could use this while shielded since it works inside another ter'angreal that cuts off the True Source. Far Madding's Guardian is an example of such a ter'angreal that inhibits access to the True Source; Aes Sedai can only channel in Far Madding with the use of a Well. It can also be used to channel inside a stedding. The only two known Wells are owned by Nynaeve al'Meara and Cadsuane Melaidhrin. Nynaeve's Well is a belt and Cadsuane's Well is the golden hummingbird hair ornament in her paralis-net. The Eye of the World was also functionally similar to a Well in that it stored the One Power within itself, but it has been depleted and it is unknown whether it could potentially be refilled.
Wells appear to differ in terms of the amount of the Power they can contain, and it is known that Nynaeve's Well has a significantly higher capacity than Cadsuane's hummingbird. To fill or refill a Well the user simply embraces saidar or seizes saidin through it as though using an angreal.