In The Wheel of Time, a female half of One Power, from one and only True Source. Just as with saidin, wielding it can produce strange effect that appears as magic to non-channelers.

Wielding saidar is described as 'surrendering' to the One Power; channeler is to embrace the flow and surrender to it, to guide the roaring river like riverbank guides river. This is obviously different from saidin, and indeed, if women tries to wield saidar in the fashion men wield saidin (seizing it), she will only find saidar fights back, the harder you try the stronger it fights back. If man, on the other hand, tried to surrender to saidin he would surely be swept away.

Unlike saidin, saidar has no problems with taint and therefore only danger on using it is power addiction, that could lead into over-drawing OP and burning yourself (and village around you) out.

P.S. It is rumored that the difference between saidin and saidar may not be that huge. While neither can actually 'see' the flows of other side (note that saidar-wielder can't 'see' even saidar before she's trained to!), they can sense it channeled. Mostly this idea is to annoy Robert Jordan, propably, though. Verin Mathwin Sedai's side comment might have something to do with it though.