From Tar Valon Library
Unless stated otherwise, all information herein is taken from A Memory of Light, Prologue.
A dreamshard is a merging of an individual's dream and Tel'aran'rhiod, almost like an offshot of Tel'aran'rhiod. The person to whom it belongs has a high degree of control. Moridin holds a meeting with the remaining chosen in his dreamshard, and it seems likely that other meetings have been held there