From the Glossary of the Wheel of Time.

Remnants of the Age of Legends that allow channeling much more of the One Power than would otherwise be possible or safe. A sa'angreal is similar to, but more powerful than, an angreal. The amount of the Power that can be wielded with a sa'angreal compares to the amount that can be handled with an angreal as the Power wielded with the aid of an angreal does to the amount that can be handled unaided. The making of them is no longer known. As with angreal, there are male and female sa'angreal. Only a handful remain, far fewer even than angreal.