ATM

1. Short for Asynchronous Transfer Mode, ATM when referring to computers is a dedicated-connection switching technology that organizes digital data into 53-byte cell units and transmits them over a physical medium using digital signal technology. ATM is capable of transmitting at speeds of 155 or 622 MBps and faster.
2. Shorthand for At The Moment, atm is commonly used in chat based communications. Below are some examples of how this could be used in chat.

I'm busy ATM.

ATM I'm watching the Simpson's.

Also see: Cell, Chat definitions, Network definitions, NSAP, NSPA, VCI