An additional slot seen on many lower-end motherboards, frequently of the MicroATX variety but becoming more prevalent on a wider range of boards. Commonly it is abbreviated as AMR or AMR Slot.
Designed by Intel, AMR is made to be a low-cost OEM solution for adding software-controlled, and hardware-assisted hardware peripherals, specifically sound cards and modems. Frequently what will happen is the digital host controller (such as a modem UART or AC '97 controller) will be on the motherboard itself, while the actual Digital to Analog Converter (DAC) will be on a card that plugs in to the AMR slot.
This is done mostly for lower cost, but also for speeding up market-production times. Separating the analog I/O functions from the motherboard can reduce production delays because the motherboard can avoid a lengthy FCC and international telecom certification process that modems, sound cards and other analogue devices are required to undergo.
Physically, AMR slots are a short (~2cm) card edge connector, beige or black in color and keyed in a fashion similar to other expansion cards as to not allow incorrect installation.