Airheads Dictionary

Announcement Traffic Indication Message (ATIM) - A unicast frame that is used in an IBSS network when Power Save Mode is enabled. If a station has buffered data for another station it will send an ATIM frame to the other station, informing it that it must stay awake until the next ATIM window so that it can receive the buffered data. Any station that either has buffered data for another station or has received an ATM will stay awake so that the buffered data can be exchanged."